There are few high-quality studies on the effectiveness of the smallpox vaccine against smallpox.
Emanuele Contini/Nuru Photo via Getty Images
One dose of one type of smallpox vaccine appears to reduce the risk of getting smallpox by about 60 percent, although this can vary depending on variants of the virus.
Cases of MPOX, formerly known as monkeypox, are rapidly increasing in the Democratic Republic of Congo, driven by a variant called lineage Ib. The vaccine used to prevent MPOX was originally developed for smallpox, and although the two viruses are related, it is unclear whether it is specifically effective against MPOX.
For more information, Sharmistha Mishra Researchers at the Institute for Clinical Evaluative Sciences in Toronto, Canada, in collaboration with colleagues, focused on a vaccine called MVA-BN, also known as JYNNEOS, Imvanex, and Imvamune, which was the most widely used smallpox vaccine in Western countries during the 2022 smallpox outbreak caused by the clade IIb variant.
Studies have shown that the effectiveness of MVA-BN for mpox varies widely, from 36% to 86%. This range is likely because the studies are observational and compare results in people of different ages, locations, and health conditions.
A randomized controlled trial is being conducted among gay, bisexual and other men who have sex with men, who made up the majority of infections in Western countries during the 2022 outbreak.
Meanwhile, Mishra’s team tried to mimic a randomized controlled trial using existing medical data. The researchers studied more than 6,000 Canadian men who were deemed to be at high risk for infection in 2022. About half received one dose of MVA-BN, and the rest did not receive the mpox vaccine. The two groups of men were matched by factors such as age and location, Mishra said.
While the official MVA-BN vaccination schedule calls for two doses administered at least 28 days apart, Mishra said the Canadian government initially opted for a one-dose vaccination protocol to spread the shots among as many at-risk people as possible.
During a follow-up period of approximately 80 days, 50 men in the unvaccinated group were diagnosed with MPOX compared with 21 in the vaccinated group, suggesting that MVA-BN reduced the risk by 58 percent.
This suggests that one dose is enough to provide a sufficient level of protection. Adam Hacker At the London Coalition for Infectious Disease Preparedness Innovations Corinne Goertz van Kessel “Scientifically, we know that two doses are more effective,” Hacker told Erasmus MC in the Netherlands.
Geerts van Kessel says the team’s approach was a good way to mimic a randomized controlled trial, but it’s unclear whether some of the men, who are older than their mid-50s, were vaccinated when smallpox was a threat, which could have influenced their immune response to the 2022 MVA-BN.
Studying how the vaccine affects the severity of disease after infection with MPOX will also help assess the vaccine’s overall effectiveness, she says.
It’s also unclear how effective the drug is specifically against lineage Ib, Goertz van Kessel says, but both she and Hacker expect MVA-BN to be at least somewhat effective against that variant as well as it is against lineage IIb, which continues to circulate in West and Central Africa.
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