One dose of a smallpox vaccine still gives good protection for mpox

One dose of smallpox vaccine appears to reduce the risk of contracting smallpox by about 60%, although this may vary depending on the variant of the virus.

The Democratic Republic of the Congo has seen a surge in cases of monkeypox (formerly known as monkeypox), caused by a strain called clade Ib. Vaccines used to protect against monkeypox were originally developed for smallpox, and while the two viruses are related, their specific effectiveness against monkeypox is unclear.

For more information, Shamisha Mishra The researcher, from the Institute for Clinical Evaluative Sciences in Toronto, Canada, and her colleagues focused on a vaccine called MVA-BN, also known as JYNNEOS, Imvanex, and Imvamune. It 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 efficacy of MVA-BN for mpox varies widely, ranging from 36% to 86%. This range may be because the studies were observational and compared results in people of different ages, locations, and health conditions.

Randomized controlled trials are being conducted among gay, bisexual and other men who have sex with men, who made up the majority of people infected in Western countries during the 2022 pandemic.

Meanwhile, Mishra’s team is trying to simulate a randomized controlled trial using existing medical data. The researchers surveyed more than 6,000 men in Canada who were considered to be at higher risk for infection in 2022. About half received one dose of MVA-BN, while the rest did not receive any MPox vaccine. Mishra said the men in the two groups were matched based on factors such as age and location.

The MVA-BN’s official protocol calls for two doses of the vaccine to be given at least 28 days apart, but the Canadian government initially opted for a single-vaccine regimen in order to spread doses to as many high-risk groups as possible, Mishra said.

During a follow-up period of about 80 days, 50 men in the unvaccinated group were diagnosed with mpox, compared with 21 men in the vaccinated group, indicating that MVA-BN reduced the risk by 58%.

This is a good level of protection for a single dose. Adam Hake At the Coalition for Epidemic Preparedness Innovations in London, Colleen Jerz Van Kessel The study was conducted at Erasmus Medical Center in the Netherlands. “From a scientific point of view, we know that two doses of the vaccine are more effective,” Hacker said.

Geurts van Kessel says the team’s approach is a good way to mimic a randomized controlled trial, but we don’t know whether any of these men in their 50s or older were vaccinated when smallpox was a threat, which could affect their immune response to MVA-BN in 2022.

Studying how the vaccine affects the severity of illness after infection with MPOX could also help us assess its overall efficacy, she said.

We also don’t know exactly how well it will work against clade Ib, Geurts van Kessel says. But she and Hacker expect MVA-BN to be at least as effective against that variant as it is against clade IIb, which is still circulating in West and Central Africa.