The paper is published in the BMJ.
Figure 1: Target trial emulation between 12 June 2022 and 27 October 2022. To learn more about how vaccine effectiveness is measured, see this study.
Summary
Why did we conduct this study?
In May 2022, there was an outbreak of mpox which became a global health emergency. To help manage the outbreak, Canada approved a vaccine called Modified Vaccinia Ankara-Bavarian Nordic (MVA-BN), which is given in two doses, 28 days apart. Early in the outbreak however, only one dose was initially given because vaccine supplies were limited. The full two-dose plan was introduced later in 2022. We sought to estimate the effectiveness of one dose of the vaccine on mpox infection.
What did we do?
Using data of 15.1 million Ontario residents, we imitated a target trial for the period June 12 to November 26, 2022, during which 691 mpox cases were reported. We focused on men aged 18 and older who had at least one syphilis test in the past year and a new diagnosis of a sexually transmitted infection (STI), or a filled prescription for HIV pre-exposure prophylaxis in the past year. We matched 3204 men who received one dose of MVA-BN vaccine with 3204 unvaccinated men. We carefully matched each individual based on factors that could act as potential confounders, to help ensure that receipt of vaccine was the only major difference between them. Each individual was followed until one of the following events: they were diagnosed with mpox, or received a second dose of the vaccine, died, or the study’s follow-up period ended on November 26, 2022. If an unvaccinated person got vaccinated during the study period, they could re-enter into the target trial as a vaccinated person and matched with a new unvaccinated individual (Figure 1).
What did we find?
We observed that 21 vaccinated individuals got mpox, compared to 50 unvaccinated individuals. This means that more than twice as many unvaccinated people got mpox. Thus, we estimated that a single dose of the MVA-BN vaccine reduced the risk of infection by 58% (95% confidence interval 31% to 75%).
What do these findings mean for public health?
While one dose of MVA-BN vaccine offers moderate protection, the full two-dose plan is still the goal as it offers the best protection. However, in cases where vaccine supply is limited, offering one dose is the next best option.
Targeted vaccination (vaccinating people at higher risk) helped control the mpox outbreak. This highlights the importance of continuing and strengthening targeted vaccination programs.
Funding
The study was funded by Canadian Immunization Research Network through the Canadian Institutes of Health Research.
Mishra Lab 