One dose of smallpox vaccine still provides good protection against mpox

Receiving one dose of one type of smallpox vaccine appears to reduce the risk of becoming infected with the smallpox virus by about 60 percent, although this can vary depending on the type of virus.

The Democratic Republic of Congo is seeing an increase in cases of mpox, formerly known as monkeypox, caused by the Ib strain. vaccine used to protect against mpox were originally developed for smallpox, and although the two viruses are related, their effectiveness specifically against mpox is unclear.

To find out more, Sharmishta Mishra at 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 mpox outbreak caused by the clade IIb variant.

Studies show that the effectiveness of MVA-BN for mpox varies widely, from 36 to 86 percent. This range may be due to the fact that the studies were observational and compared the results of people of different ages, locations, and health statuses.

Randomised controlled trials are currently underway among gay, bisexual and other men who have sex with men, who accounted for the majority of infections in Western countries during the 2022 outbreak.

Meanwhile, Mishra’s team tried to simulate a randomized controlled trial using existing medical data. The researchers studied more than 6,000 men in Canada who were considered at high risk of infection in 2022. About half received one dose of MVA-BN, while the rest did not receive any mpox vaccine. The men in the two groups were matched for factors such as age and where they lived, Mishra says.

The official MVA-BN vaccine schedule calls for two doses to be given at least 28 days apart, but the Canadian government initially chose a single-dose protocol to distribute doses to as many at-risk people as possible, Mishra said.

During an observation period of about 80 days, 50 men in the unvaccinated group were diagnosed with MPCS compared with 21 men in the vaccinated group, suggesting that MVA-BN reduced the risk by 58 percent.

This is a good level of protection for a single dose, scientists say. Adam Hacker at the Coalition for Epidemic Preparedness Innovations in London and Corinne Geerts van Kessel at Erasmus MC in the Netherlands. “From a scientific point of view, we know that two doses will be more effective,” says Hacker.

Geerts van Kessel says the team's approach was a good way to mimic a randomised controlled trial, but we don't know whether some of the men aged 55 and over were vaccinated against smallpox when it was a threat, which could have affected their immune response to MVA-BN in 2022.

Studying how the vaccine might affect the severity of illness after mpox infection will also help us assess its overall effectiveness, she said.

We also don’t know how well it works against clade Ib, says Geerts van Kessel. But both she and Hacker expect MVA-BN to be at least as effective against this variant as it is against clade IIb, which continues to circulate in West and Central Africa.