Single Dose of MVA-BN Smallpox Vaccine Shows Moderate Efficacy Against Mpox Infection

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New research demonstrates that 1 dose of the Modified Vaccinia Ankara-Bavarian Nordic (MVA-BN; Bavarian Nordic) smallpox vaccine is moderately effective in preventing infection with mpox.^1,2 The study was published in The BMJ and the authors are optimistic that this finding can help with accessibility and the distribution of vaccines, considering that mpox was declared a global public health emergency by the World Health Organization (WHO)—for the second time within 3 years—in August 2024.³

At the time of WHO’s announcement 2 mpox vaccines, LC16 (KM Biologics) and Jynneos (Bavarian Nordic), were approved in Congo under emergency use listings; however, there is currently no immunization plan established. There is only a small fraction of vaccines available (15,000 Jynneos doses were donated) but it is not enough for the 10 million doses needed to sufficiently control the outbreak. Additionally, these vaccines—as of August 2022—are not available in pharmacies, emphasizing a need for additional resources.³

For this study, the investigators enrolled 9803 men aged 18 years and older with a history of being tested for syphilis and with a laboratory-confirmed bacterial sexually transmitted infection (STI) within the previous year, or who filled a prescription for HIV pre-exposure prophylaxis within the previous year. Participants received a single dose of MVA-BN, and the outcome of interest was polymerase chain reaction-confirmed mpox infection based on the specimen collection date. Individuals were classified as “vaccinated” over 14 days after administration. Additionally, the investigators noted that estimates of the efficacy of a second dose were unable to be drawn, as few received more than 1 dose.²

Every day between June 12, 2022, and October 27, 2022, the participants who had not been vaccinated 15 days previously were matched with unvaccinated men by age, geographical region, past HIV diagnosis, number of bacterial STI diagnoses in the past 3 years, and receipt of any non-MVA-BN vaccine in the previous year. Further, the majority of enrolled patients were from Toronto, Canada.²

According to the findings, of the total 9803 men enrolled, 272 had received an mpox diagnosis during the study’s period. Of this subpopulation, 15 were hospitalized because of mpox. At a median follow-up of 85 days (IQR: 32-110 days) following the first dose among vaccinated individuals and 86 (IQR: 31-111) days among unvaccinated individuals, there was a total of 71 infections. Of these, 21 were in the vaccinated group (0.09/1000 person days, 95% confidence interval [CI] 0.05-0.13) and the remaining 50 in the unvaccinated group (0.20/1000 person days, 0.15-0.27) over the 153-day study period. Additionally, the hazard ratio for infection in the vaccinated group compared with unvaccinated group was 0.42 (95% CI 0.25-0.69), with the estimated vaccine efficacy for 1 dose of MVA-BN against mpox infection being approximately 58% (95% CI 31%-75%).²

The study findings indicate that a single dose of MVA-BN vaccine has moderate effectiveness in the prevention of mpox infection. The investigators said that to confirm this association, they found that MVA-BN was not correlated with a reduced rate of mpox infection during the first 14 days following vaccination and before developing an adequate antibody response. They also did observe that there was no protection observed for STIs, which they noted was “not expected.”²

“Our findings strengthen the evidence that MVA-BN is effective at preventing mpox infection and should be made available and accessible to communities at risk,” said the study authors.¹

REFERENCE

1. BMJ Group. One dose of smallpox vaccine moderately effective in preventing mpox infection. News release. September 11, 2024. Accessed September 20, 2024. https://www.eurekalert.org/news-releases/1057415

2. Navarro C, Lau C, Buchan SA, et al. Effectiveness of modified vaccinia Ankara-Bavarian Nordic vaccine against mpox infection: emulation of a target trial. BMJ 2024; 386 :e078243 doi:10.1136/bmj-2023-078243

3. McGovern, G. World Health Organization Declares New Mpox Outbreak a Global Public Health Emergency. Pharmacy Times. August 16, 2024. Accessed September 20, 2024. https://www.pharmacytimes.com/view/world-health-organization-declares-new-mpox-outbreak-a-global-public-health-emergency