One Dose of MVA-BN Moderately Effective Against Mpox

A single dose of modified vaccinia Ankara–Bavarian Nordic (MVA-BN) vaccine is moderately effective in preventing mpox infection, according to findings from an emulated target trial.

Analyzing information from several databases that included some 9800 adult men in Ontario, Canada, researchers found that one dose of MVA-BN was almost 60% effective in preventing mpox infection.

“Data on this vaccine’s effectiveness with just a single dose are useful for mobilizing vaccine efforts and in designing and implementing a multipronged prevention strategy to prevent outbreaks or reduce the size of outbreaks when they occur,” senior author Sharmistha Mishra, MD, PhD, associate professor of medicine at the University of Toronto, Toronto, Ontario, Canada, told Medscape Medical News.

“The vaccine should be made available and accessible to people, communities, and regions experiencing high risks of ongoing mpox transmission,” Mishra added.

The study was published on September 11 in The BMJ.

Real-World Effectiveness

The aim of the current study was to estimate the real-world effectiveness of MVA-BN. In Ontario, MVA-BN was introduced in June 2022 as postexposure prophylaxis for high-risk contacts and pre-exposure prophylaxis for gay, bisexual, and other men who have sex with men, and sex workers at high risk of exposure to mpox.

MVA-BN was approved in Canada as a series of two doses 28 days apart. A dose-sparing strategy was used initially because of concerns about a limited vaccine supply. Candidates could only receive one dose to ensure that it could be administered to the largest number of at-risk patients.

“When the mpox outbreak expanded into Canada, vaccines were made available and rapidly rolled out. The rollout of the first dose happened quickly to try and prevent as many infections as possible. There were no randomized clinical trials at the time to measure how well the vaccine worked, hence our study,” said Mishra.

The researchers used several linked databases that included laboratory, vaccination, reportable diseases, and health administrative data in their analysis.

The study population consisted of 9803 men aged ≥ 18 years. Eligible participants had a history of being tested for syphilis and a laboratory-confirmed bacterial sexually transmitted infection (STI) in the previous year or had filled a prescription for HIV pre-exposure prophylaxis in the previous year. The study period was from June to November 2022. During this time, mpox was diagnosed in 691 people in Ontario.

On each day from June 12, 2022, to October 27, 2022, men who had been vaccinated 15 days earlier were matched 1:1 with men who were unvaccinated, according to age, geographical region, past HIV diagnosis, number of bacterial STI diagnoses in the previous 3 years, and receipt of any non–MVA-BN in the previous year. A total of 3204 men who received the vaccine were matched to 3204 unvaccinated controls. The median age of matched participants was 35 years, and more than half of the participants (66.1%) were residents of Toronto.

During a median follow-up of 85 days after the first dose among vaccinated individuals and 86 days among unvaccinated individuals, there were 71 mpox infections, including 21 in the vaccinated group (0.09 per 1000 person days) and 50 in the unvaccinated group (0.20 per 1000 person days) over the 153-day study period.

The hazard ratio for infection in the vaccinated group compared with the unvaccinated group was 0.42, for an estimated vaccine effectiveness for a single dose of MVA-BN of 58%.

“In combination with other measures to help prevent infection, such as awareness of developing symptoms and reducing your contact when you have an infection, [MVA-BN] can have a really large impact,” said Mishra. She expressed the hope that resources would be made available in Canada to encourage patients who have gotten their first dose to get their second dose. She also called for the vaccine to rapidly be made available and accessible in African countries experiencing large outbreaks of mpox.

Effective Study Design

“This study uses an emulation of target trial design, which has been developed over the past decade to overcome some of the biases common to observational studies,” Stephen A. Hoption Cann, PhD, clinical professor of medicine at the University of British Columbia in Vancouver, told Medscape Medical News. “While not as robust as a randomized trial, it has been shown to be an improvement over typical observational studies.” Hoption Cann was not involved in the study.

One of the main limitations of this study is the lack of information about why one at-risk population would not take the vaccine. It is unclear whether the factors that influenced that decision could bias the outcome of laboratory-confirmed mpox infection, said Hoption Cann.

The authors were “diligent” in matching these two groups using the information from linked population databases to reduce confounding, he added. “While this matching was likely effective in reducing bias, it also eliminated a good segment of the at-risk population,” said Hoption Cann. Only 65% of the eligible population was included in the final cohort, and 71 outcomes, which was only 10% of all mpox diagnoses in the region where the study was carried out, were analyzed.

“The investigators’ access was limited to routinely collected data, and as a result, they didn’t have information on previous smallpox vaccination, sexual exposures, and individual social determinants of health,” said Hoption Cann. “However, I believe their rigorous matching likely reduced biases. That there was some moderate effectiveness during an outbreak when most individuals only received one dose is reassuring.”

The study was supported by ICES, which is funded by an annual grant from the Ontario Ministry of Health and the Ministry of Long-Term Care. Funding also was provided by the Canadian Immunization Research Network through a grant from the Public Health Agency of Canada and the Canadian Institutes of Health Research. Mishra and Hoption Cann reported no relevant financial relationships.