Single dose of mpox vaccine effective in preventing infection, study finds

Toronto, ON, September 11, 2024 — A single dose of the Modified vaccinia Ankara-Bavarian Nordic (MVA-BN) was 58% effective in protecting again mpox infection, according to a new study published in BMJ. 

Researchers from ICES, Public Health Ontario, and the MAP Centre for Urban Health Solutions at St. Michael’s Hospital of Unity Health Toronto have conducted a target trial emulation to estimate the effectiveness of the mpox vaccine. 

During the mpox outbreak in 2022, Ontario, Canada introduced the vaccine as a protective measure for individuals at high risk of exposure to mpox, which disproportionally affected gay, bisexual, and other men who have sex with men. “The initial vaccination campaign used a single dose to reach as many people at risk as possible,” says lead author Dr. Christine Navarro, a physician with Public Health Ontario. 

Using health data to mimic a randomized controlled trial 

“We used real-world health data to estimate the vaccine’s effectiveness against mpox infection as rigorously as possible,” says ICES Senior Scientist Dr. Jeff Kwong. 

Leveraging Ontario’s health data infrastructure, the study included adult men with a history of being tested for syphilis and a bacterial sexually transmitted infection (STI) in the previous year, or who filled a prescription for drugs taken to prevent getting HIV. Eligible men who were vaccinated were matched to unvaccinated men based on age, area of residence, past HIV diagnosis, number of STI diagnoses, and the receipt of any non-MVA-BN vaccine, and the infection rates in the two groups were compared to determine the effectiveness of the vaccine. 

Key Findings 

• The study included a total of 3204 men who received the MVA-BN vaccine and matched them to 3204 unvaccinated men. 

• During follow-up, 21 mpox infections were diagnosed in the vaccinated group and 50 infections diagnosed in the unvaccinated group. 

• The estimated vaccine effectiveness for a single dose of MVA-BN against mpox infection was 58%. 

“The findings of our study strengthen existing evidence that a single dose of the mpox vaccine is moderately effective against infection,” says senior author Dr. Sharmistha Mishra, an infectious disease physician and scientist with ICES and the MAP Centre for Urban Health Solutions at St. Michael’s Hospital. “The vaccine should be made rapidly available and accessible to populations at risk given the ongoing public health emergency.” 

The WHO recently declared a second public health emergency over mpox as the virus spreads rapidly across countries in Africa. In the Democratic Republic of Congo alone there have been 18,000 cases of mpox and at least 600 deaths.  

The study “Effectiveness of modified vaccinia Ankara-Bavarian Nordic vaccine against mpox infection: target trial emulation” was published in BMJ. 

Authors: Navarro C, Lau C, Buchan SA, Burchell AN, Nasreen S, Friedman L, Okpokoro E, Austin PC, Tan DHS, Gubbay JB, Kwong JC, Mishra S. 

ICES is an independent, not-for-profit research and analytics institute that uses population-based health information to produce knowledge on a broad range of healthcare issues. ICES leads cutting-edge studies and analyses evaluating healthcare policy, delivery, and population outcomes. Our knowledge is highly regarded in Canada and abroad and is widely used by government, hospitals, planners, and practitioners to make decisions about healthcare delivery and to develop policy. For the latest ICES news, follow us on X, formerly Twitter: @ICESOntario 

FOR FURTHER INFORMATION PLEASE CONTACT: 

Misty Pratt 
Senior Communications Associate, ICES 
Misty.Pratt@ices.on.ca