One Bavarian Nordic smallpox shot 58% effective against Mpox, shows study

Bavarian Nordic A/S’s smallpox vaccine was moderately effective in preventing mpox infection after a single dose, according to a study in Ontario, Canada, where the shot probably helped curb a 2022 outbreak.

One vaccination provided about 58 per cent protection against mpox infection, researchers found in the study. It was conducted from mid-June to late October 2022 among gay and bisexual men — the group most affected during a global outbreak sparked by the clade IIb strain of the monkeypox virus. 

Health experts are trying to gauge the effectiveness of Bavarian Nordic’s MVA-BN vaccine (also known as Imvamune in Canada and Jynneos in the US) and older inoculations after the spread of a potentially more dangerous clade Ib monkeypox virus strain in central Africa triggered a global health emergency last month. The vaccines, developed to protect against smallpox, a related disease, haven’t been evaluated for mpox in randomised, controlled trials — the gold standard for assessing a pharmaceutical product’s effectiveness. 

“In the absence of randomised clinical trials, our findings strengthen the evidence that MVA-BN is effective at preventing mpox infection and should be made available and accessible to communities at risk,” scientists from the Canadian Immunization Research Network Provincial Collaborative Network said in the study, published Wednesday in The BMJ. 

Previous studies have observed an effectiveness rate ranging from 36 per cent to 86 per cent across different populations, the researchers said.

Although Imvamune is approved in Canada as a series of two doses 28 days apart, Ontario initially employed a dose-sparing strategy in which patients initially received one dose owing to supply concerns. 

A two-dose regimen began there on Sept. 30, 2022. But the researchers were unable to estimate the effectiveness of two doses as only a few individuals had received a second by the end of the study period.

“Given the moderate effectiveness of a single dose, achieving high coverage with a full course could be important to prevent and manage ongoing transmission globally and prevent a large resurgence,” they said.