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

Receiving a single dose of one type of smallpox vaccine seems to lower the risk of catching mpox by around 60 per cent, although this may vary depending on the variant of the virus.

Cases of mpox, formerly known as monkeypox, are surging in the Democratic Republic of the Congo, driven by a variant called clade Ib. The vaccines used to protect against mpox were originally developed for smallpox, and while the two viruses are related, their efficacy for mpox specifically is unclear.

To learn more, Sharmistha 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. This was the most widely used smallpox vaccine in Western countries during the 2022 mpox outbreak, which was caused by the clade IIb variant.

Research suggests that MVA-BN’s efficacy for mpox varies hugely, from 36 per cent to 86 per cent. That range could be due to the studies being observational and comparing the outcomes of people with different ages, locations and health statuses.

Randomised control trials are under way 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.

In the meantime, Mishra’s team has attempted to mimic a randomised controlled trial by taking advantage of existing medical data. The researchers looked at more than 6000 men in Canada who were deemed to be at high risk of an infection in 2022. Around half received one dose of MVA-BN, while the rest hadn’t received any mpox vaccine. Men in the two groups were matched by factors such as age and location, says Mishra.

MVA-BN’s official regimen calls for two doses administered at least 28 days apart, but the Canadian government initially opted for a single vaccine protocol to spread doses out to as many at-risk people as possible, says Mishra.

Over a follow-up period of around 80 days, 50 men in the unvaccinated group were diagnosed with mpox, compared with 21 in the vaccinated group, which suggests that MVA-BN lowered the risk by 58 per cent.

This is a good level of protection for a single dose, according to Adam Hacker at the Coalition for Epidemic Preparedness Innovations in London and Corine Geurts van Kessel at Erasmus MC in the Netherlands. “Scientifically, we know that two doses would have a higher efficacy,” says Hacker.

Geurts van Kessel says the team’s approach was a good way of mimicking a randomised controlled trial, but we don’t know whether some of the men in their mid-50s or older had been 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 once infected with mpox would also help us assess its overall efficacy, she says.

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