New research indicates that a single dose of the smallpox vaccine significantly reduces the risk of contracting mpox—formerly known as monkeypox—by approximately 60%. However, the effectiveness may vary based on the viral variant in question.
Currently, the Democratic Republic of the Congo is experiencing a rise in mpox cases driven by clade Ib variant. The smallpox vaccines, originally developed to combat smallpox, are being explored for their efficacy against mpox. Despite the relationship between the two viruses, the exact effectiveness of these vaccines for mpox remains uncertain.
A team at the Institute for Clinical Evaluative Sciences in Toronto has been examining the MVA-BN vaccine, commonly known as JYNNEOS. This vaccine was primarily administered during the 2022 mpox outbreak caused by clade IIb variant in Western countries.
Current research shows that MVA-BN’s efficacy against mpox can range from 36% to 86%. This variability may stem from the observational nature of the studies, which involved participants of different ages, locations, and health statuses.
Randomized controlled trials are currently underway focusing on gay, bisexual, and other men who have sex with men, representing the majority of infections observed during the 2022 outbreak.
In an effort to simulate a randomized controlled trial, researchers analyzed medical data from over 6,000 Canadian men identified as high-risk for mpox in 2022. Approximately half received a dose of MVA-BN, while the rest did not receive any mpox vaccine. The groups were carefully matched based on age and location.
The official vaccination schedule for MVA-BN includes two doses given at least 28 days apart; however, the Canadian government initially recommended a single-dose strategy to maximize vaccination coverage among at-risk populations.
During an 80-day follow-up period, 50 individuals in the unvaccinated group were diagnosed with mpox, compared to 21 in the vaccinated group, indicating that MVA-BN reduced the risk of infection by 58%.
This level of protection from a single dose is promising, experts noted, although further efficacy is expected from the complete two-dose regimen.
Additionally, researchers acknowledged that while this approach provides valuable insights, there are gaps in knowledge regarding the vaccination history of older participants, which could influence their immune response. Understanding how the vaccine impacts the severity of illness for those infected with mpox is also crucial for evaluating overall efficacy.
While there are uncertainties about MVA-BN’s performance against the clade Ib variant specifically, experts remain hopeful that its effectiveness will be comparable to its performance against the clade IIb variant, which remains prevalent in West and Central Africa.
