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Subunit vaccine

A subunit vaccine is a type of that contains one or more purified components, or antigens, from a —such as proteins, , or peptides—rather than the entire , designed to elicit a targeted without the risk of causing . These vaccines focus on the specific parts of the that the recognizes as foreign, thereby stimulating the production of antibodies and T-cell responses to protect against the disease. Unlike live-attenuated or inactivated vaccines, subunit vaccines avoid using whole pathogens, making them safer for individuals with compromised . The development of subunit vaccines emerged in the early as an advancement over earlier vaccine types, with vaccines—such as those for and , which use inactivated bacterial toxins as subunits—being among the first examples developed in the and licensed in . The breakthrough in technology in the 1980s led to the first modern protein subunit vaccine, the (Recombivax HB), approved by the FDA in 1986, which uses yeast-produced viral surface antigens that self-assemble into virus-like particles. Subsequent innovations included acellular pertussis vaccines in the , replacing whole-cell versions to reduce side effects, and the human papillomavirus (HPV) vaccine in 2006, utilizing virus-like particles derived from the L1 protein. More recently, protein subunit vaccines have been pivotal in responses to emerging threats, such as the authorized in 2021 and included in updated formulations as of 2025, which employs a recombinant stabilized in a prefusion conformation. Subunit vaccines are typically produced through recombinant expression systems, such as , , or mammalian cells, where the target is genetically engineered, purified, and often combined with adjuvants to enhance . This production method allows for precise control over the , enabling scalability and consistency, though it may require multiple doses or boosters due to potentially weaker initial immune stimulation compared to live vaccines. Common examples in routine include the , acellular pertussis components in DTaP, and the shingles vaccine Shingrix, which uses a E from varicella-zoster virus. One key advantage of subunit vaccines is their high safety profile, as they contain no live or whole pathogens, minimizing risks of reversion to or allergic reactions in sensitive populations. They also offer stability for storage and transport, which is beneficial in resource-limited settings. However, their can be limited, often necessitating adjuvants like aluminum salts or novel ones such as AS01 in Shingrix to boost responses, and they may not induce as broad cellular immunity as whole-organism vaccines. Despite these challenges, ongoing research into advanced delivery systems, such as nanoparticles and mRNA-encoded subunits, continues to expand their efficacy against complex pathogens like and .

Fundamentals

Definition and Principles

Subunit vaccines are a class of immunizations that incorporate purified antigenic components derived from a , such as proteins, , or peptides, rather than the entire . These components are selected specifically to provoke an capable of conferring protection against the disease-causing agent without introducing the risk of from the whole pathogen. The core principles of subunit vaccines revolve around targeting immunogenic epitopes—distinct molecular regions on the that are recognized by the —to elicit focused through production and cellular immunity via T-cell activation. Unlike whole-pathogen approaches, subunit vaccines are non-replicating, which enhances their safety profile by eliminating the potential for pathogen revival or unintended replication in the host, making them suitable for immunocompromised individuals. Adjuvants are often incorporated to amplify these responses, compensating for the inherently lower of isolated subunits compared to intact organisms. Subunit vaccines differ fundamentally from other vaccine modalities in their composition and mode of . In contrast to live-attenuated vaccines, which use weakened pathogens that replicate in the host to mimic natural infection, subunit vaccines pose no risk of reversion to or causation. They also diverge from inactivated vaccines, which employ killed whole pathogens and may retain extraneous components that could trigger unwanted reactions, by focusing solely on purified antigens for a more targeted effect. Unlike nucleic acid vaccines, such as mRNA or DNA types, subunit vaccines deliver pre-formed antigens directly rather than instructing host cells to produce them, avoiding concerns related to genetic material integration or . The concept of subunit vaccines was first conceptualized in the early as a safer alternative to whole-cell vaccines, with initial developments like bacterial toxoids in the paving the way for purified strategies that gained prominence in the mid-20th century amid advances in .

Mechanism of Action

For protein and subunit vaccine antigens, which are the most common type, these exogenous materials are taken up by antigen-presenting cells (APCs) such as dendritic cells and macrophages through or . Within APCs, these antigens undergo in endosomal compartments, where they are degraded into smaller s by proteases. These peptides are then loaded onto (MHC) class II molecules and presented on the surface to CD4+ T helper cells, initiating adaptive immune activation. For MHC class I presentation, which is crucial for CD8+ activation, subunit antigens often require cross-presentation pathways where exogenous material is routed to the for proteasomal degradation before loading. antigens, in contrast, typically engage B cells directly in a T cell-independent manner, though conjugates link them to proteins for T cell involvement (see Types section for details). This primarily elicits a through CD4+ T helper activation, which promotes differentiation into plasma cells that secrete antigen-specific , such as IgG targeting surface proteins. Subunit vaccines also induce cellular responses, including CD4+ T proliferation and production, but typically generate limited CD8+ T cell responses without additional enhancements, as the exogenous nature of favors MHC II over MHC I pathways. Adjuvants play a key role in overcoming this limitation by boosting ; for instance, they promote release (e.g., IL-1β, IL-6) and APC maturation, enhancing antigen uptake and co-stimulatory molecule expression. Aluminum-based adjuvants like specifically stimulate a Th2-biased response, favoring production and IgG1/IgE class switching while modestly supporting CD4+ T help. The activated immune cascade leads to the formation of immunological memory, with long-lived memory B cells and T cells persisting after initial to enable rapid recall responses upon re-exposure. Memory B cells provide sustained production, while memory + and limited + T cells contribute to quicker effector functions, ensuring durable protection. efficacy hinges on the selection of immunogenic epitopes that elicit broadly protective responses, such as neutralizing antibodies targeting conserved spike proteins to block host cell entry. For example, protein subunit vaccines focusing on the receptor-binding domain of the generate potent neutralizing antibodies that inhibit attachment and .

Types

Protein Subunit Vaccines

Protein subunit vaccines consist of purified proteins or protein fragments derived from pathogenic organisms, serving as the primary antigens to induce protective immunity without introducing the whole . These antigens are commonly sourced from surface proteins that play key roles in -host interactions, such as glycoproteins or bacterial toxins. A prominent example is the recombinant spike protein from , utilized in vaccines like NVX-CoV2373, which targets the virus's receptor-binding domain to elicit neutralizing antibodies. Similarly, the human papillomavirus ( incorporates virus-like particles (VLPs) assembled from the L1 protein, mimicking the native structure to prevent oncogenic infections. Production of these vaccines relies on recombinant techniques, where the genetic sequence encoding the target protein is expressed in heterologous systems such as , cells, or mammalian cells to yield high quantities of the (detailed in the and Manufacturing section). This approach ensures the proteins fold into their native conformation, preserving critical structural features essential for . The surface (HBsAg), a 22-nm particle, exemplifies this process; it is expressed in and self-assembles into non-infectious VLPs that display conformational epitopes analogous to those on the . Acellular pertussis vaccines further illustrate this by incorporating purified bacterial proteins like and filamentous , detoxified and combined to broaden protection against . The hepatitis B vaccine marked a historic milestone as the first licensed protein subunit vaccine, approved in 1986 as Recombivax HB, transitioning from plasma-derived to recombinant production to eliminate risks of blood-borne contaminants. This vaccine elicits robust humoral responses by targeting conformational epitopes on HBsAg, leading to high-titer anti-HBs antibodies that confer long-term immunity in over 95% of healthy adults after three doses. In general, protein subunit vaccines stimulate B-cell activation and antibody production against these three-dimensional epitopes, which are crucial for viral neutralization and opsonization of bacterial components, though they may induce weaker T-cell responses compared to live vaccines. Key advantages of protein subunit vaccines include their exceptional , allowing storage at temperatures for extended periods—up to four years in some formulations—without loss of potency, which simplifies global distribution. Their well-defined composition also facilitates rigorous and during , reducing batch-to-batch variability. However, a primary challenge is their inherently low due to the absence of pathogen-associated molecular patterns, often requiring adjuvants like aluminum salts or AS01 to amplify innate immune signaling and enhance both and cellular responses. Despite this, their safety profile remains superior, avoiding risks of reversion to seen in live vaccines.

Polysaccharide Subunit Vaccines

Polysaccharide subunit vaccines are composed of purified capsular extracted from the outer layers of bacterial cell walls, particularly from encapsulated pathogens. These carbohydrates serve as key factors by enabling to evade , and their involves purification processes to remove impurities while preserving antigenic . A prominent example is the 23-valent (PPSV23, also known as Pneumovax 23), which includes 25 μg of purified capsular polysaccharide from each of 23 serotypes of , such as 1, 4, 6B, 14, and 19F, formulated in isotonic saline with phenol as a . Similarly, the quadrivalent meningococcal polysaccharide vaccine (MPSV4, formerly Menomune) contained 50 μg each of purified polysaccharides from serogroups A, C, Y, and W-135 of . These vaccines induce a T-cell-independent immune response, primarily activating B cells through cross-linking of B-cell receptors without requiring T-cell assistance, leading to rapid production of antibodies. The resulting humoral response is dominated by IgM and IgG2 subclasses, which facilitate opsonization and complement for bacterial clearance, but it lacks class switching to other IgG subclasses or significant IgA production. This T-independent type 2 mechanism does not generate robust immunological memory, resulting in short-lived plasma cells and minimal long-term protection upon re-exposure. Polysaccharide subunit vaccines have been applied primarily against invasive diseases caused by encapsulated bacteria, targeting pathogens like S. pneumoniae that cause pneumonia, bacteremia, and meningitis in adults and older children. PPSV23, for instance, is recommended for immunocompetent adults aged 65 years and older, as well as those with certain risk factors, demonstrating efficacy in reducing invasive pneumococcal disease by 60–70% in healthy adults. MPSV4 was used for outbreak control and travel-related protection against , providing short-term immunity in adolescents and adults. These vaccines are particularly valuable in resource-limited settings where conjugate alternatives may be unavailable. Despite their utility, polysaccharide subunit vaccines face significant challenges, including poor in infants and young children under 2 years due to immature marginal zone B cells and limited splenic function, often resulting in negligible responses. Booster doses typically fail to elicit anamnestic responses and may even induce hyporesponsiveness, diminishing over time, especially in the elderly or immunocompromised populations. Age-related declines in immune competence further reduce their protective impact, with dropping to below 50% in high-risk children, highlighting the need for enhanced formulations to achieve broader T-cell involvement.

Conjugate Vaccines

Conjugate vaccines are a type of subunit vaccine formed by the covalent chemical linkage of bacterial capsular polysaccharides to immunogenic carrier proteins, such as diphtheria toxoid (DT), tetanus toxoid (TT), or the non-toxic diphtheria toxin mutant CRM197. This conjugation process creates a hybrid antigen that combines the pathogen-specific B-cell epitopes from the polysaccharide with T-cell epitopes from the carrier protein, addressing the poor immunogenicity of polysaccharides alone, particularly in infants and young children whose immune systems respond inadequately to T-cell-independent antigens. The primary mechanism of conjugate vaccines involves transforming the immune response from T-cell-independent, which typically elicits short-lived IgM antibodies without memory cell formation, to a T-cell-dependent response. This shift occurs because the carrier protein is processed by antigen-presenting cells, allowing T-helper cells to recognize its peptides and provide signals for B-cell activation, proliferation, and differentiation. As a result, conjugate vaccines promote immunoglobulin class switching to IgG subclasses, affinity maturation in germinal centers, and the generation of long-lived plasma cells and memory B cells, leading to sustained protection and booster responses upon re-exposure. These vaccines have been pivotal in preventing invasive bacterial diseases, serving as the foundation for immunizations against type b (Hib), , and . For instance, Hib conjugate vaccines, which link the polyribosyl ribitol phosphate (PRP) polysaccharide to carriers like or outer membrane protein complex (OMPC), have virtually eliminated invasive Hib disease in vaccinated populations. Similarly, meningococcal conjugate vaccines target serogroups A, C, W, and Y, while pneumococcal conjugate vaccines like PCV13 cover 13 serotypes of pneumococcal capsular polysaccharides conjugated to 197, reducing invasive pneumococcal disease by over 90% in children under 5 years. Conjugate vaccines emerged in the as a solution to the limitations of plain vaccines, which failed to induce protective immunity in children under 2 years due to immature immune responses; the first Hib conjugate vaccine was licensed in the United States in 1987. The now recommends their inclusion in routine childhood immunization programs worldwide, with administration in a 3- or 4-dose schedule starting at 6 weeks of age, contributing to over 90% reduction in global Hib burden since widespread adoption. The selection of carrier protein is critical, as overuse of the same carrier (e.g., CRM197 in multiple vaccines) can lead to carrier-induced epitopic suppression, where pre-existing anti-carrier antibodies reduce responses to the , potentially impacting overall vaccine efficacy and benefits from reduced nasopharyngeal carriage.

Peptide Subunit Vaccines

Peptide subunit vaccines utilize short synthetic peptide sequences, typically comprising 8 to 20 , designed to mimic specific immunogenic epitopes from pathogens, such as T-cell or B-cell epitopes. These peptides can be linear or cyclic, with cyclic forms often enhancing stability and mimicking the conformational structure of native epitopes more effectively. For instance, peptides derived from parasite proteins, like those targeting T-cell epitopes in , have been synthesized to focus on precise antigenic regions without including extraneous pathogen material. A key advantage of peptide subunit vaccines lies in their high specificity, which allows for targeted immune responses against defined s, minimizing off-target effects and reducing the risk of allergic reactions associated with whole-pathogen vaccines. Their enables rapid production, high purity, and straightforward , making them cost-effective and suitable for therapeutic applications, particularly in where personalized targeting is beneficial. Additionally, these vaccines exhibit an excellent safety profile, with low toxicity and no risk of causing , as they lack replicative components. Despite these benefits, peptide subunit vaccines face significant challenges, primarily their inherently poor , which often fails to elicit robust immune responses without enhancement. This limitation arises from the small size of peptides, which hinders efficient by antigen-presenting cells and proper for MHC , frequently necessitating the co-administration of adjuvants or advanced delivery systems like nanoparticles to boost efficacy. Conformational constraints also pose issues, as linear peptides may not correctly to replicate the three-dimensional of native epitopes, potentially reducing to immune receptors. In applications, peptide subunit vaccines have been explored experimentally for challenging infectious diseases, including , where multi-epitope peptides target conserved viral regions to induce broad T-cell responses. For , epitope-based designs have focused on glycoprotein peptides to elicit protective antibodies and cytotoxic T cells in preclinical models. candidates, such as those using peptides from circumsporozoite protein epitopes, demonstrate potential for multi-epitope strategies that provide coverage against diverse parasite strains. These approaches often incorporate multiple peptides to achieve broader . The evolution of peptide subunit vaccines has been markedly advanced since the through the development of peptide libraries, which enable of immunogenic sequences, and computational design tools that predict structures and interactions with MHC molecules. Early efforts relied on empirical , but of bioinformatics and structural modeling has allowed for rational optimization, leading to more effective multi- constructs and improved formulations. This progression has shifted peptide vaccines from basic prophylactic concepts toward sophisticated therapeutic platforms, particularly for chronic infections and cancers.

Production and Manufacturing

Antigen Identification and Selection

Antigen identification and selection represent the foundational step in subunit vaccine development, where specific immunogenic components of a pathogen are chosen to elicit protective immunity without the risks associated with whole-pathogen vaccines. This process integrates empirical experimental techniques and computational tools to pinpoint antigens that can effectively stimulate B-cell and T-cell responses. Key methods include epitope mapping through structural biology approaches like X-ray crystallography, which resolves the atomic-level interactions between antigens and antibodies to identify neutralizing epitopes, as demonstrated in studies on viral surface proteins. Complementing this, enzyme-linked immunosorbent assays (ELISA) enable high-throughput screening of antibody binding to potential antigens, while animal challenge studies validate protective efficacy by assessing survival rates and immune correlates in vivo models. These methods collectively ensure the selection of antigens capable of inducing targeted, long-lasting immunity. Selection criteria emphasize antigens that are highly conserved across strains to minimize escape by variants, prominently exposed on the surface for accessibility to the , and non-toxic to avoid adverse reactions in hosts. Additionally, candidates must steer clear of regions prone to immune evasion, such as hypervariable loops in envelopes that facilitate antigenic drift. For instance, surface exposure is prioritized using bioinformatics predictions of transmembrane domains and extracellular localization signals, which have proven effective in identifying vaccine targets for intracellular s. Non-toxicity is assessed through checks against host proteins to prevent , ensuring the antigen's safety profile aligns with regulatory standards. Computational tools accelerate this pipeline by predicting immunogenic epitopes from genomic data. The Immune Epitope Database (IEDB) serves as a primary resource for analyzing B-cell and T-cell epitopes, integrating experimental data to forecast binding affinities and scores for novel candidates. Reverse vaccinology, a genomics-driven approach, scans entire genomes to select surface-exposed proteins with properties and low similarity to human proteins, as pioneered in development and extended to other bacteria and viruses. This method has identified over 600 potential antigens in , highlighting its efficiency in prioritizing candidates for empirical validation. Representative examples illustrate these principles in practice. For subunit vaccines, the was selected due to its surface exposure, role in host cell entry, and conservation in receptor-binding domains, enabling broad neutralization across variants in candidates like Novavax's NVX-CoV2373. In bacterial conjugate vaccines, capsular from were chosen for their and strain coverage, conjugated to carrier proteins to enhance T-cell help and overcome T-independent responses. These selections underscore the balance between specificity—targeting key pathogenic mechanisms—and breadth to counter strain diversity. A primary challenge in selection lies in reconciling broad against diverse variants with sufficient specificity to avoid non-protective or reactogenic responses. Highly conserved antigens may elicit weaker responses due to , while variable ones risk obsolescence from mutations, as observed in evolving lineages where escape reduces efficacy. Addressing this requires iterative refinement, combining multi-antigen formulations to expand coverage without diluting potency.

Recombinant Expression Systems

Recombinant expression systems are biotechnological platforms that utilize genetically engineered host organisms to produce subunit vaccine , enabling scalable manufacturing of purified proteins or virus-like particles (VLPs). These systems involve inserting the gene encoding the target into the host's or a vector, followed by expression under controlled conditions to yield high quantities of the recombinant protein. The choice of system depends on the 's structural requirements, such as the need for proper folding, bond formation, and post-translational modifications like , which are critical for and stability. Bacterial systems, particularly , offer high yields and low production costs due to rapid growth and simple cultivation in inexpensive media. These prokaryotic hosts excel in expressing simple, non-glycosylated proteins but lack the machinery for eukaryotic post-translational modifications, often resulting in misfolded or insoluble proteins that require refolding steps. For instance, attempts to produce (HBV) surface antigen () in E. coli yield non-glycosylated particles with reduced compared to eukaryotic systems, highlighting limitations for glycoproteins. Despite these challenges, bacterial expression is suitable for antigens or when modifications are unnecessary, achieving titers up to several grams per liter in optimized strains. Yeast systems, such as and Pichia pastoris, provide eukaryotic advantages including proper , secretion into the culture medium for easier purification, and basic patterns, while maintaining relatively low costs and high expression levels. S. cerevisiae has been pivotal for the HBV vaccine, where recombinant self-assembles into immunogenic VLPs with yields of 10-20 mg/L, demonstrating effective disulfide bond formation absent in . S. cerevisiae, which is utilized in the human papillomavirus (HPV) vaccine , produces L1 capsid proteins that form VLPs mimicking native virions for robust antibody responses. These systems balance scalability with moderate post-translational capabilities, though their differs from mammalian patterns, potentially affecting antigenicity. Mammalian cell lines like ovary (CHO) and human embryonic kidney 293 (HEK293) cells deliver the most authentic post-translational modifications, including complex N-linked essential for proteins with intricate structures, ensuring native-like folding and . CHO cells, widely used for biopharmaceuticals, produce the zoster subunit vaccine Shingrix's E (gE) at titers exceeding 1 g/L in serum-free bioreactors, enabling high efficacy against through proper sialylation and stability. HEK293 cells support for rapid prototyping, as seen in hemagglutinin (HA) subunit production, where they facilitate scalable yields in suspension culture while replicating for enhanced vaccine potency. However, these systems incur higher costs due to complex media and slower growth, limiting their use to antigens demanding precise modifications. Insect cell systems employing baculovirus vectors, such as in Sf9 or High Five cells, enable rapid, high-level expression of complex eukaryotic proteins with post-translational modifications closer to mammalian profiles, including glycosylation and phosphorylation, at intermediate costs. The baculovirus expression vector system (BEVS) drives strong promoters for transient production, yielding up to 30 mg/L of influenza HA for the FluBlok vaccine, where the antigen's trimeric structure and sialic acid modifications elicit broad hemagglutination inhibition responses. This platform's versatility supports VLP formation and is advantageous for seasonal vaccines requiring quick adaptation, though potential for hyper-glycosylation necessitates optimization. Following expression in any system, the crude antigen proceeds to purification to achieve pharmaceutical-grade purity. Selection of the expression system is guided by antigen complexity: prokaryotic hosts like E. coli suffice for simple, unmodified peptides, while eukaryotic systems— for moderately complex antigens like HBV , insect cells for viral glycoproteins like , and mammalian cells for highly demanding ones like —are chosen to ensure functional, immunogenic products without compromising yield or cost-effectiveness.

Purification and Formulation

Following the production of subunit antigens through recombinant expression systems, purification involves downstream processing to isolate the target antigen from host cell components, ensuring high purity and removal of potential contaminants. Common techniques include affinity chromatography, which exploits specific interactions between the antigen and ligands such as monoclonal antibodies or tags (e.g., His-tag) to achieve purities often exceeding 95%, ion-exchange chromatography for separating based on charge differences, and ultrafiltration for concentration and size-based separation. These methods are sequentially applied to minimize impurities like host cell proteins (HCPs) and DNA, with regulatory guidelines requiring host cell DNA levels below 10 ng per dose to mitigate risks of oncogenicity or immunogenicity. Once purified, antigens are formulated by integrating to enhance , as subunit vaccines often require immune stimulation for robust responses. Aluminum-based adjuvants like promote Th2-biased responses by facilitating uptake and sustained release, while advanced systems such as AS01—a liposome formulation containing monophosphoryl (MPL) and Quillaja saponaria 21 (QS-21)—induce a balanced Th1/Th2 profile and enable dose-sparing, reducing requirements by up to 10-fold in some cases. Adjuvant selection depends on the target ; for instance, is widely used in vaccines for production, whereas AS01 improves T-cell responses in vaccines. Formulation further includes buffering to maintain stability (typically 6.5–7.5 for protein ), addition of stabilizers like sugars (e.g., ) or to prevent aggregation, and lyophilization (freeze-drying) for long-term storage, which can extend shelf-life to 2–3 years at 2–8°C by removing water and preserving antigen structure. For multi-valent vaccines, such as those targeting multiple HPV types, purified antigens are mixed under controlled conditions to ensure compatibility and uniform potency without cross-interference. These steps prioritize thermal stability, with lyophilized forms often showing less than 10% potency loss after 6 months at elevated temperatures. Quality control encompasses rigorous testing to verify product and , including sterility assays via membrane or direct to confirm absence of viable microorganisms, and potency assessments using enzyme-linked immunosorbent assay () to quantify content against reference standards, ensuring levels meet predefined thresholds (e.g., ≥80% of labeled amount). Additional checks for purity, such as or HPLC, confirm removal of HCPs below 100 ppm, while endotoxin levels are limited to <5 EU/dose via Limulus amebocyte lysate testing. These assays are validated per ICH guidelines and performed on final lots to support batch release. Subunit vaccines are predominantly formulated for injectable delivery, typically intramuscular or subcutaneous routes to optimize systemic immunity, with preservatives like thimerosal added for multi-dose vials. Emerging approaches include microneedle patches for painless, self-administered transdermal delivery, which have shown comparable to injections for subunit vaccines by targeting skin-resident antigen-presenting cells.

Advantages and Limitations

Advantages

Subunit vaccines offer a superior profile compared to live-attenuated vaccines, as they contain no viable components, eliminating the risk of reversion to or incomplete inactivation during manufacturing or administration. This makes them particularly suitable for immunocompromised individuals, who may not tolerate vaccines with live elements due to the potential for adverse reactions. The production of subunit vaccines leverages standardized methods, enabling large-scale manufacturing that is both consistent and cost-effective relative to traditional vaccine platforms requiring pathogen . These methods facilitate scalability by utilizing systems, such as bacterial, , or mammalian cells, to produce antigens without the complexities of handling infectious agents. By focusing exclusively on key immunogenic antigens, subunit vaccines elicit targeted immune responses that minimize extraneous reactions to non-essential pathogen components, thereby enhancing specificity and potentially simplifying pathways to regulatory approval through well-characterized, purified formulations. This precision reduces the likelihood of off-target immune activation seen in whole-pathogen vaccines. Some subunit vaccines exhibit , allowing storage at ambient temperatures without significant loss of potency, which streamlines and distribution in low-resource settings where cold-chain is limited. For instance, certain recombinant protein formulations maintain efficacy after prolonged exposure to elevated temperatures, addressing a key barrier to vaccine access in developing regions. Ethically, subunit vaccines avoid the need for propagating virulent during production, thereby reducing risks associated with high-containment facilities and minimizing environmental concerns related to pathogen handling. This approach supports safer, more accessible development globally.

Disadvantages

Subunit vaccines generally possess poor inherent , as they consist of isolated antigens that fail to replicate the full structure necessary for strong immune activation, often requiring adjuvants to boost responses and potentially leading to weaker cellular immunity compared to live-attenuated vaccines. This limitation stems from the antigens' inability to mimic natural infection pathways, resulting in predominantly antibody-mediated responses rather than comprehensive T-cell activation. Development of subunit vaccines is complex and resource-intensive, particularly in antigen selection and ensuring proper protein folding, which can lead to misfolded products with reduced efficacy and extend the timeline to licensure to 10-15 years due to iterative testing and optimization. These challenges arise from the need to identify immunodominant epitopes that maintain conformational integrity during recombinant expression, often necessitating advanced bioinformatics and trial-and-error approaches. Their limited breadth of protection poses another drawback, as subunit vaccines targeting specific antigens may not effectively counter antigenic drift in evolving pathogens, such as viruses, where mismatches between vaccine strains and circulating variants reduce effectiveness. Unlike whole-pathogen s, they typically do not induce mucosal immunity, further restricting their utility against respiratory or enteric infections. Production costs for subunit vaccines are high, primarily due to the reliance on recombinant systems like mammalian or insect cell cultures, which demand expensive media, bioreactors, and downstream purification processes compared to simpler manufacturing. Yields can vary widely (e.g., 1-10 g/L in cells), but scalability issues and for complex proteins elevate overall expenses. Regulatory requirements add further hurdles, mandating rigorous testing for purity, potency, and consistency to ensure the absence of contaminants and reliable , which prolongs approval and increases development burdens. These standards, enforced by agencies like the FDA, emphasize lot-release assays for biologic activity, often requiring animal models or advanced analytics to verify each batch's performance.

Safety Profile

Adverse Effects

Subunit vaccines are generally well-tolerated, with the most common adverse effects being mild and transient local reactions at the injection site, including pain, soreness, redness, and swelling, alongside systemic symptoms such as , , low-grade fever, and . These effects typically onset within hours to days post-vaccination and resolve spontaneously within 1-2 days without intervention. For instance, in clinical and post-marketing data for the recombinant , injection site pain affects 3% to 29% of recipients, while and swelling occur in about 3%, and systemic symptoms like fever are reported in 1% to 6%. Similar profiles are seen with the human papillomavirus (HPV) vaccine, where pain, redness, or swelling at the site affects up to 90% of recipients, often accompanied by , , or in 10-20% of cases. Adjuvant use in subunit vaccines can amplify local reactogenicity, particularly through enhanced . The AS04 , a combination of monophosphoryl lipid A (MPL) and aluminum hydroxide employed in certain HPV like , is associated with more prolonged injection site pain and swelling compared to aluminum-only adjuvanted formulations, though these remain mild and self-limiting. Aluminum-based adjuvants in protein subunit , such as those in formulations, may also contribute to transient , , or in a subset of recipients, reflecting their role in stimulating innate immune responses. In contrast, non-adjuvanted subunit exhibit lower rates of these local inflammatory responses due to their reduced immunostimulatory potency. Rare serious adverse events with subunit vaccines include reactions such as , occurring at an estimated rate of less than 1 per million doses across licensed products. For the , the incidence of is approximately 1.1 per million doses, with a likely causal association in individuals sensitive to yeast-derived components. Post-licensure surveillance systems like the (VAERS) in the United States and global networks monitor these events, confirming that mild reactions occur in 10-20% of doses for vaccines like , while severe events remain exceedingly uncommon. Importantly, extensive reviews have found no established causal link between subunit vaccines and autoimmune disorders or Guillain-Barré syndrome in the majority of cases, with observed incidences aligning with background population rates.

Contraindications and Precautions

Subunit vaccines, being non-live, have few absolute contraindications, primarily limited to individuals with a history of severe allergic reaction, such as , to a vaccine component or following a previous dose of the same . For yeast-derived subunit like the and certain HPV (e.g., and Gardasil 9), a history of hypersensitivity to yeast is also an absolute . Precautions warrant caution but do not preclude unless risks outweigh benefits. These include moderate or severe acute illness, with or without fever, where should be deferred until recovery to avoid confusing vaccine-related symptoms with those of the illness. is not an absolute for most ; available data, including and experience, show no evidence of risk. For example, the is considered safe during with no adverse fetal outcomes reported in available studies. However, HPV vaccines are not recommended during due to insufficient safety data, and any inadvertently administered doses should be reported to manufacturers for monitoring. In individuals with , subunit vaccines are generally safe to administer as they pose no risk of vaccine-derived , unlike live vaccines. Nonetheless, immunocompromised patients may exhibit reduced immune responses, potentially necessitating higher doses, additional boosters, or serologic monitoring to confirm immunogenicity, as seen with vaccination in those with or undergoing . Drug interactions with subunit vaccines are minimal, and no specific spacing is required when co-administering with live vaccines, though general best practices recommend vaccinating during stable health periods. For special populations, conjugate subunit vaccines, such as those for type b (Hib), are safe and routinely recommended for infants starting at 2 months of age. In the elderly, subunit vaccines like the adjuvanted recombinant (Shingrix) require precautions for those with acute illness but are otherwise indicated, often with booster considerations to maintain efficacy. The Centers for Disease Control and Prevention (CDC) and (WHO) emphasize pre-vaccination screening for allergies and acute conditions, with providers trained to recognize and manage rare anaphylactic risks through immediate access to epinephrine. WHO guidelines further stress that mild illnesses, such as low-grade fever or , do not constitute contraindications for subunit vaccines, promoting broad access while prioritizing safety in vulnerable groups.

Licensed Examples

Hepatitis B Vaccine

The recombinant hepatitis B subunit vaccine targets the hepatitis B surface antigen (HBsAg), a key viral protein expressed using recombinant DNA technology in yeast cells such as Saccharomyces cerevisiae. This approach produces noninfectious HBsAg particles that mimic the natural virus structure to elicit an immune response without the risks associated with live or whole-virus vaccines. The first such vaccine, Recombivax HB, was licensed by the U.S. Food and Drug Administration in 1986, marking a pivotal advancement in subunit vaccine technology by shifting from plasma-derived antigens to safer, scalable recombinant production. The recommended schedule for the subunit vaccine consists of three intramuscular doses at 0, 1, and 6 months, which induces protective anti-HBs antibodies in greater than 95% of healthy adults, conferring long-term seroprotection against infection. This regimen is highly effective in preventing both acute and chronic (HBV) disease, with seroprotection rates remaining above 90% for at least 20-30 years in most recipients. Universal infant , initiated globally following recommendations, has dramatically reduced chronic HBV infections by approximately 90% in vaccinated birth cohorts, averting millions of cases of liver and . Available formulations include pediatric doses (typically 5 mcg HBsAg per 0.5 mL for infants and children) and adult doses (10 mcg HBsAg per 1 mL), allowing age-appropriate administration while maintaining equivalent . Combination vaccines, such as Twinrix, integrate the hepatitis B subunit antigen with inactivated virus components for dual protection in a single 3- or 4-dose series, particularly useful for travelers and high-risk adults. As of 2025, no major formulation changes have occurred for the core recombinant vaccines, though vaccination strategies continue to evolve for high-risk groups, including expanded universal adult recommendations for ages 19-59 years and targeted catch-up for those 60 and older with risk factors like .

Human Papillomavirus (HPV) Vaccine

The human papillomavirus () vaccines represent a key application of subunit vaccine technology, utilizing virus-like particles (VLPs) derived from the L1 major capsid protein to mimic the structure of the capsid without incorporating DNA, thereby inducing a robust against oncogenic and low-risk HPV types. These VLPs self-assemble from recombinant L1 proteins expressed in systems such as or cells, providing non-infectious antigens that target the primary causes of cervical and other HPV-related cancers. Several variants of HPV VLP vaccines have been developed, differing in the number of HPV types covered and adjuvant composition. The bivalent vaccine, , targets HPV types 16 and 18—responsible for approximately 70% of cervical cancers—and incorporates the , which combines aluminum hydroxide with monophosphoryl lipid A to enhance T-cell mediated immunity and duration of protection. In contrast, the quadrivalent formulation, originally licensed as in 2006 by the (FDA), protects against HPV types 6, 11, 16, and 18, addressing both high-risk oncogenic types and those causing . An updated nonavalent version, , approved by the FDA in December 2014, extends coverage to nine types (6, 11, 16, 18, 31, 33, 45, 52, and 58), potentially preventing up to 90% of HPV-related cervical cancers. Clinical trials and real-world evidence demonstrate high efficacy of these vaccines, with protection exceeding 90% against persistent infection, precancerous lesions (such as grades 2 and 3), and vaccine-type cervical cancers when administered prior to HPV exposure. For instance, the quadrivalent vaccine showed nearly 100% efficacy against HPV 16/18-related high-grade cervical lesions in phase III trials, while the nonavalent version achieved 96% efficacy against high-grade cervical disease caused by additional targeted types in HPV-naïve populations. Bivalent exhibited 92.9% efficacy against HPV 16/18-associated grade 2 or higher in per-protocol analyses. The recommended vaccination schedule targets adolescents to maximize pre-exposure immunity, with a two-dose series (administered 6–12 months apart) for individuals initiating between ages 9 and 14, and a three-dose series (at 0, 1–2, and 6 months) for those starting at age 15 or older. This approach ensures strong responses, with rates approaching 100% in clinical studies across variants. By 2025, widespread HPV vaccination has significantly reduced the prevalence of vaccine-targeted HPV types and related in vaccinated populations, with U.S. data showing an 80% decrease in higher-grade cervical precancer incidence among women aged 20–24 screened during 2008–2022, reflecting effects even among unvaccinated individuals. Globally, similar trends have emerged in high-uptake regions, contributing to progress toward elimination goals set by the .

Influenza Vaccine

Recombinant protein subunit vaccines represent a key advancement in immunization, utilizing purified (HA) proteins produced through technology in insect cell systems, such as those used in Flublok. These vaccines target the HA surface , which facilitates viral attachment to host cells, and are formulated without the use of eggs, thereby avoiding potential adaptation issues or allergic reactions associated with traditional egg-based production. Unlike whole-virus or split-virus formulations, subunit versions focus solely on HA to elicit a targeted , with Flublok containing three times the standard HA dose per strain for enhanced . The primary challenge in developing these vaccines lies in the annual antigenic drift of viruses, necessitating biannual strain selection by the (WHO) based on global surveillance data to predict circulating variants. For the 2025-2026 northern hemisphere season, WHO recommends trivalent formulations incorporating an A(H1N1)pdm09-like , an A(H3N2) , and a B/Victoria lineage , reflecting the obsolescence of the B/Yamagata lineage and a shift away from quadrivalent vaccines. This process ensures vaccines match dominant strains, though mismatches can occur due to post-selection viral evolution. against culture-confirmed in matched strains typically ranges from 40% to 60% in adults, with reduced protection—around 45%—observed in older adults aged 65 and above due to and lower responses. In the elderly, recombinant HA vaccines like Flublok demonstrate relative efficacy improvements of up to 30% compared to standard inactivated vaccines in those over 50. The transition to recombinant subunit influenza vaccines accelerated in 2013 with the FDA approval of Flublok, marking the first fully recombinant option and providing a safer alternative for individuals with severe egg allergies by eliminating egg-derived components entirely. Clinical trials preceding approval showed 44.6% efficacy against in adults, supporting its role in seasonal prophylaxis. By 2025, these vaccines remain a cornerstone of influenza prevention strategies, with ongoing efforts toward universal influenza vaccines—such as those targeting conserved viral epitopes—still in clinical development, while recombinant HA-based subunit formulations continue as the standard for annual updates.

Other Notable Vaccines

Shingrix, a recombinant subunit vaccine targeting herpes zoster (), consists of the E derived from varicella-zoster virus combined with the AS01B system. Licensed by the U.S. (FDA) in October 2017 for adults aged 50 years and older, it demonstrated vaccine efficacy of 97.2% against herpes zoster in individuals aged 50-59 years and 97.4% in those aged 60 years and older in pivotal phase 3 trials involving over 38,000 participants. Additionally, it showed 89.3% efficacy against , a common complication of shingles, highlighting its role in reducing severe outcomes in older adults. Real-world studies have confirmed sustained effectiveness, with vaccine effectiveness reaching 51.8% to 57.7% against herpes zoster in previously vaccinated adults as of 2025. Novavax's COVID-19 vaccine, known as Nuvaxovid or NVX-CoV2373, is a protein nanoparticle subunit vaccine featuring the SARS-CoV-2 spike protein stabilized in prefusion conformation, adjuvanted with Matrix-M to enhance immune responses. Initially authorized for emergency use by the FDA in 2021 and fully approved in May 2025 for the 2025-2026 formula targeting updated variants, it exhibited 90.4% efficacy against symptomatic COVID-19 caused by the original strain in a phase 3 trial of approximately 30,000 adults. By 2025, formulations have been expanded to address variants like Omicron sublineages, with booster doses showing robust humoral responses and up to 80% effectiveness against hospitalization in real-world settings. This vaccine represents a non-mRNA alternative, particularly valued for its stability and compatibility with existing cold-chain infrastructure. Among bacterial subunit vaccines, Prevnar 13 (PCV13) is a 13-valent comprising purified capsular from serotypes conjugated to the CRM197 carrier protein, enabling T-cell dependent immune responses. Licensed by the FDA in 2010 for children and extended to adults in 2011, it demonstrated approximately 90%-97% against vaccine-type invasive pneumococcal disease in infants, based on post-licensure studies, and 45-46% against vaccine-type in adults aged 65 and older in clinical studies. Similarly, Menveo is a quadrivalent meningococcal targeting serogroups A, C, Y, and W-135, with linked to CRM197. Approved by the FDA in 2010 for individuals aged 2 months through 55 years, it induced seroprotective responses in 89-96% of infants after a two-dose series, correlating with high against invasive . A notable advancement in respiratory vaccines is Arexvy, a subunit vaccine for () prevention in older adults, utilizing the prefusion-stabilized F adjuvanted with AS01E. Licensed by the FDA in May 2023 for individuals aged 60 years and older, it showed 82.6% efficacy against -associated lower disease in the first season in a phase 3 trial of over 25,000 participants, with sustained protection of 62.9% over three seasons. By 2025, it has been integrated into routine recommendations for at-risk elderly populations, reducing hospitalizations by 60-65%. Subunit vaccines are increasingly incorporated into combination formulations to simplify immunization schedules and enhance coverage, as seen in multivalent pneumococcal and meningococcal products that target multiple serotypes simultaneously. Therapeutic applications are also expanding, with subunit designs explored for chronic conditions like cancer and infectious disease management, projecting a market growth to $86.8 billion by 2035 driven by adjuvanted and innovations.

Historical Development

Early Discoveries

The development of subunit vaccines traces its roots to early 20th-century efforts to create safer alternatives to whole-pathogen vaccines by isolating specific protective components. In the , researchers developed toxoid vaccines, such as the toxoid, by inactivating bacterial toxins with formalin to retain without toxicity; this approach, pioneered by Ramon and colleagues, marked an initial form of subunit vaccination and was widely used by the mid-1930s. Similarly, in the 1930s, work by and others at the Rockefeller Institute isolated pneumococcal capsular , demonstrating their ability to induce type-specific antibodies, which laid the groundwork for polysaccharide-based subunit vaccines tested in clinical trials during that decade and the 1940s. The 1960s and 1970s saw pivotal advances with the emergence of technology, enabling precise production of viral and bacterial s. Key milestones included the creation of the first recombinant DNA molecules in 1972 by and the development of techniques by Stanley Cohen and in 1973, which opened pathways for genetically engineered subunit vaccines. Concurrently, in the 1970s, the purification of hepatitis B surface (HBsAg) from plasma of asymptomatic carriers provided the basis for the first plasma-derived subunit vaccine against , licensed in 1981 under the leadership of at . This vaccine was later transitioned to a recombinant form produced in yeast cells, approved in 1986, to eliminate risks associated with human plasma. In the 1980s, early trials of conjugate subunit vaccines advanced the field, particularly for type b (Hib), where were linked to carrier proteins like diphtheria to enhance in infants; the first such , PRP-D, underwent clinical testing starting in 1986 and was licensed in 1987. These developments highlighted the critical need for adjuvants, recognized since the with alum's use in and formulations to boost weak immune responses typical of purified subunit antigens. The push for subunit vaccines during this era was largely driven by the epidemic, which caused significant global morbidity and mortality in the 1970s, and the subsequent AIDS crisis in the 1980s, which underscored the dangers of plasma-derived products due to transmission risks and accelerated the adoption of recombinant technologies.

Key Milestones and Evolution

The advent of technology in the revolutionized subunit vaccine development, enabling the production of safer s without live or whole pathogens. In July 1986, the U.S. (FDA) approved Recombivax HB, the first recombinant surface vaccine developed by Merck using cells, marking a pivotal shift from plasma-derived versions and addressing global concerns over blood-borne transmission. This was followed by advancements in s for ; in December 1987, the FDA licensed ProHIBiT, the first type b (Hib) , which linked s to carrier proteins for enhanced immunogenicity in children under two years, dramatically reducing invasive Hib disease incidence. The 1990s saw further progress with (VLP) technology, culminating in early trials for human papillomavirus (HPV) vaccines, though full approvals came later. The 2000s brought innovations in s and broader applications. In 2007, the approved , the first using the AS04 adjuvant system (monophosphoryl plus aluminum hydroxide), which enhanced T-cell responses and provided longer-lasting protection against HPV-16/18 compared to aluminum-only formulations. The FDA followed with approval in 2009. By 2013, recombinant technology extended to with the FDA's approval of Flublok, a trivalent subunit vaccine produced in cells expressing proteins, offering an egg-free alternative that improved production scalability during pandemics. The 2010s and 2020s highlighted subunit vaccines' resilience amid emerging threats. In October 2017, the FDA approved Shingrix, a recombinant using E with the AS01B , demonstrating over 90% in preventing herpes zoster in adults aged 50 and older, surpassing live-attenuated options. The accelerated subunit vaccine deployment; in July 2022, the FDA granted to Novavax's protein-based , utilizing nanoparticle-displayed with Matrix-M , providing a non-mRNA option that maintained relevance despite mRNA vaccines' rapid rollout. Subunit approaches persisted due to their established safety profile and adaptability. Regulatory frameworks evolved to support global access. The FDA's fast-track designation, formalized under the 1997 FDA Modernization Act, expedited reviews for pandemic vaccines like Novavax's, allowing rolling submissions and priority evaluation to address unmet needs. Similarly, the World Health Organization's prequalification program, expanded in the , vetted subunit vaccines such as recombinant and Hib conjugates for UN procurement, ensuring quality and affordability in low-income countries. Subunit vaccines have evolved from single-antigen formulations, like early , to multivalent designs targeting multiple strains or pathogens for comprehensive protection. Examples include the progression of HPV vaccines from bivalent (2007) to 9-valent Gardasil 9 (2014), covering additional oncogenic types. By 2025, integration in antigen design, using for prediction and protein optimization, has accelerated development of next-generation multivalent subunit vaccines, reducing timelines from years to months.

Research Directions

Current Challenges

One major challenge in subunit vaccine development is addressing immunogenicity gaps, particularly against rapidly evolving viral variants. The emergence of variants such as , characterized by over 30 mutations in the including the receptor-binding domain, has significantly reduced the neutralizing efficacy of first-generation subunit vaccines targeting the receptor-binding domain, with significant reductions, often 4- to 30-fold compared to the original strain. This immune escape, driven by the virus's high of 0.8–2.38 × 10⁻³ substitutions per site per year, complicates the design of broadly protective subunit vaccines, often requiring multivalent or formulations to elicit cross-reactive immunity. Similarly, antigenic variability in other pathogens like necessitates universal vaccine strategies, yet current subunit approaches struggle to provide durable protection across diverse strains without frequent reformulation. Access and equity remain critical barriers, especially in low- and middle-income countries (LMICs), where high production costs and dependencies hinder widespread deployment. Despite the relative of subunit vaccines compared to mRNA platforms, cold-chain requirements for and exacerbate logistical challenges in resource-limited settings, contributing to uneven distribution and lower vaccination coverage. The disproportionate in LMICs amplifies these inequities, as technical, manufacturing, and funding risks limit affordable access, with global vaccine importers like nations relying heavily on external supplies. Regulatory delays pose another hurdle, particularly in harmonizing global standards for novel s essential to enhancing subunit vaccine potency. New adjuvants must undergo rigorous for , , and duration of , often facing scrutiny over insufficient manufacturing data or potential excessive immune activation, which prolongs approval timelines across jurisdictions. This fragmented regulatory landscape, including varying requirements for safety assessments, impedes the rapid integration of innovative adjuvant systems into subunit platforms. Lessons from the highlight tensions between development speed and safety, alongside persistent vulnerabilities. Subunit vaccines like experienced delays due to manufacturing scale-up issues and hurdles, underscoring the need for balanced to avoid compromising efficacy while accelerating responses. disruptions revealed dependencies on global infrastructure, with bottlenecks in raw materials and distribution amplifying inequities during crises. Data gaps persist regarding long-term in diverse populations, with post-2020 revealing insufficient on sustained across groups, ethnicities, and comorbidities. Most trials focus on short-term in healthy adults, leaving uncertainties about waning immunity and real-world effectiveness in vulnerable cohorts like the elderly or immunocompromised. Enhanced post-licensure monitoring is needed to address these voids and inform booster strategies.

Emerging Technologies and Future Prospects

Advancements in technology are enhancing the efficacy of subunit vaccines through next-generation immunostimulators and integrated delivery systems. (TLR) agonists, such as CpG oligodeoxynucleotides, activate TLR9 on antigen-presenting cells to promote a Th1-biased , which is crucial for eliciting robust cellular immunity against intracellular pathogens in subunit formulations. These have demonstrated improved titers and T-cell responses in preclinical models of subunit vaccines, addressing limitations in traditional alum-based systems that favor Th2 responses. Self- nanoparticles further innovate by combining with built-in adjuvant activity, such as lipid or polymeric nanoparticles incorporating TLR ligands, which enhance antigen uptake and without requiring separate adjuvant components. Clinical trials of these systems, including saponin-based Matrix-M™ for subunit antigens, have shown up to 10-fold increases in neutralizing levels compared to non-adjuvanted controls. Delivery innovations are expanding subunit vaccine accessibility and potency beyond injectable formats. mRNA-encoded subunit vaccines enable transient expression of specific antigens, leveraging mRNA's rapid manufacturability while maintaining the targeted of protein subunits. Viral vectors, such as modified adenoviruses, facilitate the expression of subunit antigens like viral spikes, offering sustained and improved mucosal immunity in respiratory models. Plant-based oral and vaccines represent a needle-free alternative, where subunit antigens are produced in transgenic plants (e.g., or rice) for ingestion, inducing systemic and mucosal responses; examples include the HIV-1 Tat protein expressed in tomatoes, which elicited immune responses including mucosal IgA in animal studies. Computational design tools are accelerating subunit vaccine development by predicting optimal immunogens. and algorithms, such as models for , identify conserved B-cell and T-cell epitopes with high accuracy, reducing experimental screening needs; for instance, tools like DeepVac have designed multi-epitope constructs for emerging viruses with predicted scores exceeding 90%. Structure-based vaccinology has been transformed by , which by 2025 enables precise prediction of structures and conformations, facilitating the rational design of stable subunit variants; 3's integration of small-molecule interactions has aided in engineering spike trimers for broader . These tools have shortened design timelines from years to months in recent subunit vaccine pipelines. Future prospects for subunit vaccines include universal formulations targeting conserved epitopes to combat antigenic drift in viruses like influenza and coronaviruses. For influenza, recombinant protein subunits focusing on the hemagglutinin stem domain, such as COBRA-based constructs or ferritin nanoparticles, have demonstrated heterosubtypic protection in preclinical studies and early-phase trials (phase 1 as of 2025) in ferrets against diverse strains. Similar approaches for coronaviruses emphasize nucleocapsid or receptor-binding domain subunits to elicit pan-sarbecovirus immunity, with preclinical data showing reduced viral loads in challenge models. Therapeutic applications are promising, particularly peptide-based subunit vaccines for , which target conserved Gag and Env epitopes to boost CD8+ T-cell responses in chronic infection, and for cancer, where neoantigen peptides personalized via prediction have induced tumor regression in phase 1 trials. The has profoundly influenced subunit vaccine evolution, accelerating protein-based platforms through streamlined regulatory pathways and manufacturing scales. Novavax's adjuvanted spike protein subunit vaccine, whose original formulation demonstrated approximately 90% efficacy against symptomatic infection in clinical trials, with updated versions authorized for the 2024-2025 season based on against current variants, exemplifies this speedup. strategies combining subunit antigens with mRNA elements are in clinical trials, such as co-formulated protein-mRNA boosters for enhanced durability, with phase 2 data indicating synergistic humoral and cellular responses against variants. These developments position subunit vaccines as versatile backbones for next-generation prophylactics and therapeutics.