Smallpox vaccine
The smallpox vaccine is a vaccine against the variola virus that causes smallpox, pioneered by English physician Edward Jenner in 1796 by inoculating humans with cowpox virus material to induce protective immunity, marking the advent of vaccination as a preventive medical practice.[1][2] Jenner drew from folk observations that milkmaids exposed to cowpox rarely contracted smallpox, testing the approach by scratching cowpox pus into the arm of eight-year-old James Phipps, who subsequently resisted deliberate exposure to smallpox matter.[3][2] This method, derived from empirical observation rather than prior variolation techniques using live smallpox, demonstrated cross-protective efficacy against the more virulent disease.[3] The vaccine's deployment evolved from rudimentary arm-to-arm transfers to standardized production, enabling mass immunization that progressively contained outbreaks and, through coordinated international efforts led by the World Health Organization from 1967 onward, achieved global eradication.[2][4] Smallpox was certified eradicated in 1980, with the last natural case occurring in 1977, representing the sole complete elimination of a human pathogen via vaccination and underscoring the intervention's causal role in terminating a disease that historically killed hundreds of millions annually.[4] While profoundly effective in averting transmission through herd immunity thresholds met in targeted campaigns, the vaccine exhibited limitations including waning protection requiring boosters and rare but severe complications such as generalized vaccinia or post-vaccinial encephalitis, occurring at rates of approximately 1-2 per million primary doses in historical data from healthy populations.[5] These risks, though mitigated by screening and ring vaccination strategies during eradication, highlight trade-offs in early vaccine technologies weighed against smallpox's 30% case-fatality rate.[5]Historical Development
Pre-Vaccination Practices: Variolation
Variolation, the deliberate inoculation of healthy individuals with material derived from smallpox lesions, emerged as an empirical risk-reduction strategy in regions including China, parts of Africa, and the Ottoman Empire by the 17th century, though some accounts trace Chinese insufflation—blowing powdered scabs into the nostrils—back to the mid-1500s or earlier.[2][6] In these practices, variolators typically scratched or abraded the skin and rubbed in pus from active pustules or crushed scabs from mild cases, aiming to induce a controlled infection that conferred subsequent protection against severe natural exposure.[6][7] This method yielded a mortality rate of approximately 1-2% among recipients, far lower than the 20-30% fatality observed in unvaccinated individuals contracting variola major naturally, yet it inherently involved live virulent virus, resulting in symptomatic illness akin to a attenuated form of the disease.[7][8][3] Protection was individual and probabilistic, dependent on unpredictable factors such as viral dose, recipient immunity, and strain virulence, without inducing sterilizing immunity that halted transmission or achieving population-level herd effects.[5] The practice reached Europe through Lady Mary Wortley Montagu, who observed it in the Ottoman Empire in 1717 and arranged variolation for her own children in 1718 before promoting it in England upon her return; by 1721, amid a London outbreak, it gained traction among elites and was trialed publicly, extending to colonial America where figures like Zabdiel Boylston performed inoculations during the Boston epidemic that year.[9][3] Despite endorsements from some physicians, variolation's causal flaws persisted: the uncontrolled viral load often escalated to full-blown disease in vulnerable recipients, and the contagious exudate from induced lesions sparked secondary outbreaks among the uninoculated, amplifying rather than containing epidemics in dense settings.[5]Edward Jenner's Breakthrough
Edward Jenner, an English physician, observed that dairymaids who had contracted cowpox—a mild disease transmitted from cattle—appeared resistant to smallpox, a pattern rooted in longstanding rural folklore he encountered in his Gloucestershire practice.[1] This empirical association prompted Jenner to hypothesize that deliberate exposure to cowpox could confer protective immunity against the more lethal variola virus without the dangers of variolation, which involved direct inoculation of smallpox material and carried a risk of inducing full disease.[10] Jenner's reasoning emphasized the antigenic similarity between cowpox and smallpox as orthopoxviruses, allowing cross-protection while cowpox's attenuated virulence minimized harm.[3] On May 14, 1796, Jenner tested this by extracting lymph from a cowpox pustule on the hand of Sarah Nelmes, a dairymaid infected by a cow named Blossom, and inoculating it into the arm of eight-year-old James Phipps, who developed a mild local reaction but recovered fully.[11] Approximately two months later, in July 1796, Jenner challenged Phipps with variola material from a smallpox lesion; the boy exhibited no symptoms of the disease, demonstrating successful cross-immunity.[2] Jenner repeated the variolation challenge on Phipps multiple times over subsequent months, consistently observing resistance, which validated the protective effect in this initial case.[12] Jenner extended these findings through additional experiments on other subjects, confirming cowpox's prophylactic efficacy before publishing his results in 1798 as An Inquiry into the Causes and Effects of the Variolae Vaccinae, a self-financed pamphlet detailing 23 cases and advocating vaccination as a safer alternative to variolation.[13] The work's publication spurred rapid adoption; within two years, vaccination was performed across Britain, with Jenner providing free inoculations to the poor from his Berkeley facility, marking the empirical foundation of modern vaccinology.[3]
Advancements in Production and Distribution
In the late 19th century, smallpox vaccine production transitioned from human arm-to-arm transfers, which carried risks of transmitting other pathogens, to large-scale cultivation in calves, enabling greater volumes and reduced contamination. This shift gained prominence in the 1880s, particularly in Europe, where calf lymph—harvested from vaccinia-infected calf skin—became the standard material for vaccine preparation.[14] In France, techniques for producing glycerinated calf lymph, which involved mixing lymph with glycerol to preserve potency and inhibit bacterial growth, were refined and widely adopted by the 1890s, further minimizing impurities compared to fresh lymph.[14] The establishment of dedicated vaccine institutes facilitated standardization and quality control prior to international coordination. In Britain, the Lister Institute of Preventive Medicine, founded in 1891 (initially as the British Institute of Preventive Medicine and renamed the Jenner Institute in 1898), pioneered consistent calf lymph production methods, supplying reliable vaccine stocks for national programs and influencing global practices through empirical testing of lymph viability and potency.[15] Similar facilities in France and Germany emphasized animal-derived lymph to ensure uniformity, with production scaling to support compulsory vaccination laws that achieved widespread coverage in Europe and North America by the early 1900s, dramatically lowering incidence rates.[14] Twentieth-century innovations addressed stability for distribution in diverse climates. In the late 1940s, British virologist Leslie Collier developed freeze-drying (lyophilization) techniques for smallpox vaccine, producing a heat-stable powder that retained efficacy without refrigeration, as demonstrated in trials where reconstituted vaccine induced protective responses comparable to fresh material.[2] This method, commercialized as Dryvax in the U.S., extended shelf life to years and enabled transport to remote areas, markedly improving logistical feasibility for large-scale immunization.[5] Administration efficiency advanced with the bifurcated needle, invented in 1965 by Benjamin Rubin at Wyeth Laboratories. This disposable, two-pronged tool required only about 0.0025 mL of vaccine per dose—roughly one-fifteenth of syringe volumes—while delivering consistent intradermal inoculation via multiple skin punctures, as validated in field tests showing take rates exceeding 95%.[16] Its simplicity reduced training needs and waste, supporting rapid vaccination drives that vaccinated hundreds of millions globally by the late 1960s.[2]Global Eradication Efforts
The World Health Organization (WHO) launched the Intensified Smallpox Eradication Program in 1967, building on prior partial efforts by prioritizing active surveillance to detect cases and ring vaccination to contain outbreaks.[17] Ring vaccination entailed immunizing immediate contacts of confirmed cases, extending to surrounding villages or communities to create barriers of immunity, which proved more resource-efficient than mass campaigns in vast endemic regions.[1] This strategy, pioneered during a 1967 outbreak in Nigeria, relied on the bifurcated needle for precise delivery and freeze-dried vaccine stability, enabling teams to vaccinate hundreds daily while tracing chains of transmission.[18] Empirical data from the campaign demonstrated that achieving 80-90% coverage within outbreak rings interrupted transmission, as modeled outbreaks showed containment when cases were identified within days and contacts vaccinated promptly, reducing effective reproduction numbers below unity.[19] In regions like West and Central Africa, initial implementations curbed resurgences, with surveillance-containment accounting for over 90% of successes by 1970, per program evaluations, rather than broad immunization alone.[20] Vaccine potency, confirmed at 95% efficacy against clinical disease in controlled settings, drove outcomes, independent of parallel public health gains like sanitation, given smallpox's primary airborne, person-to-person spread unaffected by water or hygiene improvements.[21] Major milestones included the elimination of endemic transmission in South Asia by 1975 and Africa by 1977, culminating in the last naturally occurring case: hospital cook Ali Maow Maalin in Merka, Somalia, on October 26, 1977, traced to a nearby outbreak and contained via ring measures.[22] Following two years of global surveillance with no detections, the WHO's Global Commission certified eradication on December 9, 1979, affirmed by the World Health Assembly on May 8, 1980.[23] Eradication averted an estimated 300 million deaths in the 20th century alone, equivalent to halting 2-5 million annual fatalities at pre-campaign rates.[24] Persistent challenges in India and Ethiopia highlighted logistical demands: India's 1974 resurgence, with over 100,000 cases amid dense rural populations and vaccine hesitancy, required vaccinating 150 million and deploying 100,000 workers, overcome through incentives like food rations and door-to-door enforcement.[25] In Ethiopia, remote highlands and civil unrest necessitated helicopter-assisted vaccine drops and mobile teams, eradicating the disease by 1976 despite terrain isolating 80% of cases.[26] These adaptations underscored vaccination's causal primacy, as incidence plummeted post-ring interventions—India's cases fell 99% from 1974 peaks by 1975—validating targeted immunity over generalized interventions.[27]Vaccine Types and Generations
First-Generation Vaccines
First-generation smallpox vaccines utilized live vaccinia virus, a poxvirus distinct from but cross-protective against variola, propagated primarily through animal inoculation such as in calves to harvest lymph material.[28] These vaccines, exemplified by Dryvax in the United States, involved infecting the scarified skin of calves with seed virus strains like New York City Board of Health, followed by extraction and lyophilization of the resulting vesicular fluid for storage and distribution.[29] Production methods relying on animal sources persisted from the 19th century into the late 20th century, enabling the manufacture of billions of doses despite inherent variability in viral yield and potential for bacterial contamination due to non-sterile harvesting processes.[14] [30] Administered via skin scarification using a bifurcated needle or lancet to introduce the virus into the dermis, these vaccines achieved take rates—successful pustule formation indicating replication—of approximately 95% among primary vaccinees, correlating with durable immunity often lasting decades.[31] [32] Empirical evidence from 19th-century field applications, including controlled outbreaks in Britain post-Vaccination Act implementation, demonstrated high protective efficacy, with vaccinated populations exhibiting markedly reduced incidence and mortality compared to unvaccinated groups.[33] Despite production inconsistencies, such as strain heterogeneity from serial passages and incomplete purification, the vaccines' robustness facilitated widespread use and contributed to empirical success in interrupting transmission chains.[34]Second-Generation Vaccines
Second-generation smallpox vaccines, developed primarily in the 1960s and 1970s, represented a shift from animal-derived production methods to cell culture or embryonated egg systems, aiming to improve vaccine purity and consistency while preserving the immunogenicity of live vaccinia virus strains. These vaccines addressed limitations of first-generation calf lymph products, such as potential contamination with adventitious agents like bacteria or other viruses from animal tissues, by employing controlled in vitro propagation. Production typically involved strains like Lister or Temple of Heaven, grown in substrates including Vero monkey kidney cells, rabbit kidney cells, or embryonated chicken eggs, which allowed for better standardization under emerging good manufacturing practices.[35][36][37] This transition facilitated more reliable large-scale manufacturing, particularly for the World Health Organization's intensified eradication campaign from 1967 onward, where second-generation vaccines contributed to vaccine supplies in regions requiring heat-stable, freeze-dried formulations less prone to production failures due to animal health variability. For instance, Soviet programs utilized embryonated egg methods to produce millions of doses, demonstrating scalability without compromising potency as measured by plaque assays or animal challenge models. Immunogenicity remained comparable to earlier vaccines, with seroconversion rates exceeding 95% in vaccinated populations, as evidenced by neutralization antibody titers equivalent to those from calf lymph-derived lots.[37][34][38] Key examples include precursors to modern cell-culture vaccines like ACAM1000, derived from historical strains and propagated in Vero cells, which maintained biological characteristics such as replication competence and protective efficacy against orthopoxviruses in preclinical models. These vaccines supported the final phases of smallpox containment in endemic areas by 1977, with reduced lot-to-lot variability enabling broader distribution through intensified vaccination strategies. Despite these advances, adoption was uneven globally, as many eradication efforts continued relying on established first-generation stocks until certification of eradication.[39][40][1]Third-Generation Vaccines
Third-generation smallpox vaccines consist of highly attenuated vaccinia virus strains engineered post-eradication to replicate poorly or not at all in human cells, thereby reducing reactogenicity while preserving immunogenicity against variola virus. These vaccines emerged in response to the need for safer stockpiles against potential reintroduction of smallpox via bioterrorism, prioritizing attenuation through extensive serial passaging or genetic deletions over the calf lymph or chick embryo methods of prior generations.[41][42] Modified Vaccinia Ankara (MVA), developed in the 1970s at Germany's Bernhard-Nocht Institute for Tropical Medicine, underwent 570 serial passages in chicken embryo fibroblasts, yielding six major genomic deletions that render it replication-deficient in most mammalian cells, including human keratinocytes and dendritic cells. This attenuation eliminates production of many virulence factors present in wild-type vaccinia, enabling transient gene expression for immune stimulation without productive infection. MVA induced protective immunity in over 120,000 German vaccinees during the 1970s eradication tail-end, with no neurotoxicity reported, and nonhuman primate studies demonstrated survival against lethal orthopoxvirus challenges comparable to first-generation vaccines.[43][44][43] LC16m8, licensed in Japan in 1975 by the Chemo-Sero-Therapeutic Research Institute, was attenuated via approximately 150 serial passages of the Lister strain in rabbit kidney cells followed by Vero cell adaptation, resulting in reduced neurovirulence and plaque size. Clinical trials in Japan from 1974–1975 administered 90,000 doses to infants and adults, achieving take rates of 94–95% after a single intradermal dose and seroconversion in most recipients, with no severe adverse events among over 100,000 pediatric uses. Animal models confirmed efficacy equivalent to the Dryvax strain against intranasal vaccinia challenge, supporting its inclusion in Japan's national stockpile for biodefense.[41][45][46] MVA-BN (marketed as Imvamune, Imvanex, or JYNNEOS), a further refined MVA clonal isolate propagated in Vero cells and selected for complete replication incompetence in human cell lines, underwent plaque purification and genetic stabilization for enhanced safety. It received European Medicines Agency approval in 2013 and U.S. FDA licensure in 2019 for smallpox prevention in high-risk adults. In 2025, the dBTF variant—derived from the vaccinia Tiantan strain with targeted deletions in BTF genes—demonstrated superior immunogenicity in preclinical models, eliciting robust antibody responses and protection against orthopoxviruses in mice and macaques, positioning it as an evolving third-generation candidate.[47][48][49]Mechanism of Action and Immunology
How the Vaccine Works
The smallpox vaccine employs live vaccinia virus, a member of the Orthopoxvirus genus closely related to variola virus, administered percutaneously via scarification using a bifurcated needle to deliver approximately 10^5 to 10^8 plaque-forming units (PFU) into the superficial layers of the skin.[50][51] Following inoculation, vaccinia virus infects and replicates within keratinocytes and dermal cells at the site, inducing localized viral propagation that culminates in a characteristic vesicular-pustular lesion, termed a "take," typically observable within 3-7 days and serving as a clinical marker of successful replication and immunogenicity.[52][53] This replication process triggers innate immune activation through pattern recognition receptors, while evading complete clearance to facilitate antigen presentation and mimic key aspects of orthopoxvirus pathogenesis without systemic dissemination in immunocompetent hosts.[51] The vaccine's protective mechanism relies on eliciting a multifaceted adaptive immune response, including vaccinia-specific neutralizing antibodies via B cells and cytotoxic CD8+ T cells alongside helper CD4+ T cells, which collectively target viral antigens and provide cross-protection against variola through conserved orthopoxvirus proteins such as hemagglutinin, DNA polymerase, and envelope glycoproteins.[52][54] These shared epitopes enable cellular and humoral effectors to recognize and neutralize variola virions and infected cells, achieving historical protection rates of approximately 95% against smallpox disease when a successful take occurs.[52][51] Immunity wanes over time, with peak antibody and T-cell responses conferring full protection for 3-5 years post-vaccination, followed by durable partial immunity mediated by long-lived memory cells that persists for decades, as evidenced by sustained neutralizing titers and T-cell functionality in longitudinal studies of vaccinated cohorts.[52][55] This temporal profile underscores the vaccine's reliance on live viral replication for robust, infection-like priming of cross-reactive adaptive responses rather than sterile humoral blockade alone.[54]Immune Response Induced
The smallpox vaccine, utilizing live vaccinia virus, induces a robust primary immune response dominated by humoral and cellular components that collectively limit viral replication and dissemination. Following vaccination, the initial humoral response involves production of IgM antibodies within the first week, transitioning to IgG isotypes that target key vaccinia structural proteins such as the hemagglutinin and A27L envelope proteins; these peak in titer approximately 2-4 weeks post-inoculation, with neutralizing antibodies forming the primary correlate for extracellular virus neutralization.[56][54] Historical data from mass vaccination campaigns document seroconversion rates exceeding 95% in immunocompetent individuals, reflecting effective B-cell activation and plasma cell differentiation.[31] Cellular immunity emerges concurrently, with CD4+ T helper cells facilitating antibody class switching and CD8+ cytotoxic T lymphocytes (CTLs) targeting intracellular vaccinia antigens via MHC class I presentation; these CTLs exhibit cross-reactivity against conserved orthopoxvirus epitopes, such as those in the D3 and E3 proteins, enabling containment of related pathogens like variola.[57][58] The vaccine's live-virus nature drives formation of long-lived memory B cells and central/effector memory T cells, which sustain low-level antibody production and rapid recall responses to prevent systemic spread upon orthopoxvirus challenge.[54] Revaccination in previously primed individuals elicits an anamnestic response, restoring neutralizing antibody titers within days and amplifying both humoral and cellular arms; protection correlates with post-boost neutralizing titers exceeding 1:20 dilution, alongside IFN-γ-producing T-cell frequencies above baseline.[59][60] This dual-arm induction underscores the vaccine's causal efficacy in orthopoxvirus clearance, beyond transient innate barriers like skin-localized interferons.[61]Efficacy
Clinical and Historical Evidence
Historical analyses of 19th-century outbreaks in the United Kingdom demonstrated the smallpox vaccine's impact on reducing incidence and mortality. In areas with high vaccination coverage, such as London, smallpox cases were markedly lower at 5.5 per 10,000 population in 1900, compared to 31.3 per 10,000 in low-vaccination Leicester, where alternative measures like isolation were emphasized but insufficient to match vaccinated regions' outcomes.[62] The Royal Commission on Vaccination, reporting in 1896, reviewed extensive data and concluded that vaccination provided effective protection against smallpox, with vaccinated individuals experiencing substantially lower attack rates and fatalities during epidemics.[63] Field observations from outbreaks quantified the vaccine's role in post-exposure prophylaxis. Vaccination administered within 3-4 days of exposure was estimated to prevent disease in 80-90% of cases, based on historical records and expert consensus from analyses of past epidemics, where timely vaccination reduced severity and mortality even after contact with variola virus.[64][65] The World Health Organization's intensified eradication program from 1967 onward relied on targeted vaccination campaigns achieving coverage levels approaching 80%, sufficient to induce herd immunity thresholds and interrupt transmission chains.[66] By the early 1970s, annual smallpox cases had declined from over 131,000 in 1967 to 33,000, correlating with millions vaccinated yearly in endemic regions, culminating in global cessation of transmission by 1977.[67] Eradication averted an estimated 2 million deaths annually that occurred prior to control efforts, with over 300 million fatalities in the 20th century alone prevented through vaccination-driven declines.[68] No randomized controlled trials were conducted due to ethical constraints in endemic settings, but robust observational data from cohort comparisons, outbreak investigations, and program evaluations consistently demonstrated mortality reductions exceeding 90% in vaccinated versus unvaccinated groups.[21][1]Effectiveness Against Smallpox and Variants
The smallpox vaccine demonstrated high effectiveness against Variola major, the more virulent strain, with historical data indicating approximately 95% protection against infection in vaccinated individuals.[69] Empirical evidence from outbreaks showed a weighted average vaccine effectiveness of 91.1% against Variola major, reflecting substantial reductions in incidence and mortality, though case-fatality rates remained elevated at around 11% among those vaccinated more than 20 years prior compared to 52% in unvaccinated cases.[70] Against Variola minor, the milder form, effectiveness approached 96.4% on average, contributing to near-complete control in affected populations.[70] Cross-protection extended to related orthopoxviruses, with historical surveillance data from 1981–1986 indicating 85% protection against human monkeypox among previously smallpox-vaccinated individuals.[71] In the 2022–2025 mpox (clade IIb) outbreaks, repurposed smallpox vaccines showed variable but significant effectiveness: JYNNEOS (a third-generation vaccine) achieved 66–89% vaccine effectiveness against disease following two doses, with observational studies estimating around 82% overall.[72][73][74] ACAM2000, a second-generation vaccine, provided robust protection in nonhuman primate models, with 100% survival against lethal challenge versus 0% in controls, though human outbreak data for mpox are more limited and suggest comparable cross-efficacy to JYNNEOS in high-risk groups.[75] Vaccine-induced protection wanes over decades, with near-complete immunity against mortality persisting for 20–30 years post-vaccination before gradual decline, driven by slowly diminishing T-cell responses (half-life of 8–15 years) despite more stable antibody levels.[54][76] Failures were rare and typically attributed to suboptimal vaccine potency, improper administration, or host factors rather than viral evasion, underscoring the vaccine's reliability under standard conditions.[5] No cross-protection exists against non-orthopoxviruses, limiting scope to related pox genera.[21]Safety Profile
Adverse Events by Vaccine Type
First- and second-generation smallpox vaccines, which utilize replication-competent vaccinia virus strains such as Dryvax or ACAM2000 derived from calf lymph or similar methods, are associated with a spectrum of adverse events ranging from mild local reactions to rare but severe complications. Surveillance data from U.S. programs in the 1960s, involving millions of doses, reported postvaccinial encephalitis at rates of 2.9 to 8.5 per million primary vaccinations, with progressive vaccinia (vaccinia necrosum) occurring in approximately 12.3 per million primary vaccinees and eczema vaccinatum in 0.7 to 39 per million, particularly elevated among individuals with atopic dermatitis. [77] [78] Generalized vaccinia, a disseminated rash, was observed at 2.9 to 52 per million primary doses, while inadvertent inoculation (accidental spread to other body sites or contacts) occurred in about 529 per million vaccinations, with transmission to close contacts at roughly 47 per million. [77] Mortality was estimated at 1 per million primary vaccinations and 0.1 to 0.25 per million revaccinations, often linked to encephalitis, progressive vaccinia, or eczema vaccinatum. [79] [80] In the U.S. pre-event smallpox vaccination program from 2002 to 2003, approximately 450,000 doses of second-generation vaccines like Dryvax and ACAM2000 were administered to military personnel and civilians, yielding 822 adverse event reports to the Vaccine Adverse Event Reporting System (VAERS) from about 38,885 civilian vaccinations alone, with 100 designated as serious, including 85 hospitalizations, 2 permanent disabilities, and 2 to 3 deaths (one from apparent cardiac complication, others potentially vaccine-related). [81] [82] Myopericarditis emerged as a notable risk, with confirmed or suspected cases at rates of 15 to 21 per 10,000 primary vaccinees in early surveillance, alongside rarer neurologic events like suspected encephalitis (3 cases) and meningitis (13 cases) in the civilian cohort. [83] [84] ACAM2000-specific data indicate suspect myopericarditis at 5.7 per 1,000 primary vaccinees (95% CI: 1.9-13.3), though broader surveillance shows lower confirmed rates, with neurologic events like encephalitis or encephalomyelitis remaining infrequent but documented. [85] [86] Third-generation vaccines, such as modified vaccinia Ankara-Bavarian Nordic (MVA-BN, also known as Imvanex or Imvamune), are replication-deficient in human cells, resulting in a markedly improved safety profile with primarily mild to moderate local reactions like injection-site pain, redness, swelling, and itching, reported in most recipients but resolving without intervention. [87] [88] Serious adverse events are rare, with no deaths or progressive vaccinia observed in clinical trials or post-licensure use; myopericarditis occurs at rates below 1 per 1,000, significantly lower than with replication-competent vaccines, and no cases of eczema vaccinatum or vaccinia transmission have been linked due to the attenuated replication. [89] [90] In real-world data from mpox vaccination campaigns, MVA-BN showed low severe event frequency, though intradermal administration may increase transient syncope compared to subcutaneous routes. [91] [92] Phase 3 trials reported eight serious events across thousands of doses, none deemed vaccine-attributable beyond expected background rates. [43]| Adverse Event | First/Second-Generation Rate (per million primary doses) | Third-Generation (MVA-BN) Rate |
|---|---|---|
| Postvaccinial Encephalitis | 2.9–8.5 [77] | Rare/none reported [89] |
| Eczema Vaccinatum | 0.7–39 [77] | None [89] |
| Myopericarditis | 15–21 per 10,000 [83] | <1 per 1,000 [90] |
| Death | ~1 [79] | None [43] |