One dose of a smallpox vaccine still provides good protection against smallpox

It appears that getting a single dose of one type of smallpox vaccine reduces the risk of getting smallpox by about 60 percent, although this can vary depending on the strain of the virus.

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

To learn more, Sharmistha Mishra of the Institute for Clinical Evaluative Sciences in Toronto, Canada, and her colleagues turned to 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 the effectiveness of MVA-BN for mpox varies widely, from 36 percent to 86 percent. This range may be due to the fact that the studies are observational, comparing outcomes across people of different ages, locations, and health statuses.

Randomized controlled trials are being conducted 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 attempted to replicate a randomized controlled trial using existing medical data. The researchers looked at more than 6,000 men in Canada who were at high risk of infection in 2022. About half received one dose of MVA-BN, while the rest did not receive the mpox vaccine. Men in the two groups were matched based on factors such as age and location, Mishra says.

According to Mishra, the official regimen of MVA-BN is to administer two doses at least 28 days apart. However, the Canadian government initially opted for a single vaccination protocol to spread the doses as evenly as possible across as many at-risk groups 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. This shows that MVA-BN reduced the risk by 58 percent.

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

According to Geurts van Kessel, the team’s approach was a good way to mimic a randomized controlled trial. However, we don’t know whether some men in their mid-50s or older had been vaccinated against smallpox when it was a threat. This could have affected their immune response to MVA-BN in 2022.

By investigating how the vaccine can affect the severity of disease after infection with mpox, we can also assess its overall effectiveness, she says.

We also don’t know how well it works specifically against clade Ib, says Geurts 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 is still circulating in West and Central Africa.