Single dose of MVA-BN vaccine offers 58% protection against mpox

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Sep 16 2024

Study confirms effectiveness of single-dose MVA-BN vaccine in reducing mpox infections in high-risk populations, highlighting the need for expanded access and targeted interventions.

Study: Effectiveness of modified vaccinia Ankara-Bavarian Nordic vaccine against mpox infection: emulation of a target trial. Image Credit: Marina Demiduik / Shutterstock.com

A recent study published in The BMJ confirms the effectiveness of the modified vaccinia Ankara-Bavarian Nordic (MVA-BN) vaccine against mpox.

Global efforts to contain mpox

Mpox was diagnosed in over 20 countries for the first time in May 2022, which led the World Health Organization (WHO) to declare a public health emergency of international concern on July 23, 2022. The WHO has recommended second or third generation vaccines against smallpox to be used to protect high-risk individuals against mpox infection.

MVA-BN is a third generation live attenuated smallpox vaccine containing non-replicating virus. This vaccine was initially administered in June 2022 in Ontario, Canada as post-exposure prophylaxis to primarily protect men who have sex with men (MSM) and sex workers against mpox infection.

Due to limited vaccine supply, only one vaccine dose was administered, despite recommendations for a second dose to be administered 28 days following the initial vaccine dose. By September 30, 2022, the two-dose program was initiated.

Observational data on the effectiveness of MVA-BN against mpox infection has indicated vaccine effectiveness between 36% and 86%. However, these data may be biased due to the presence of confounding factors.

About the study

The current study estimated the vaccine effectiveness of MVA-BN in a broader high-risk population using carefully matched cohorts to emulate a trial setting. The study included 9,803 men aged 18 years and older who had been tested for syphilis over the last year.

All study participants were diagnosed with at least one bacterial sexually transmitted infection (STI) in the same period, which was confirmed by laboratory testing or a recorded prescription for pre-exposure prophylaxis for human immunodeficiency virus (HIV) during the previous year.

All study participants were administered the subcutaneous MVA-BN vaccine between June 12, 2022, and October 27, 2022. The rates of laboratory-confirmed mpox infections were then compared between the study cohort and unvaccinated men 15 days after vaccination.

June 12, 2022, the start of the study period, corresponded to the initial availability of pre-exposure vaccination. Comparatively, October 27, 2022, reflected when the outbreak began to wane, at which point 691 individuals were diagnosed with mpox in Ontario.

The two cohorts were matched for age, location, history of HIV, frequency of bacterial STIs over the past three years, and for any other mpox vaccine received over the previous year.

Vaccine effectiveness

A total of 3,204 men were included in both the vaccinated and unvaccinated cohorts. During the study period of 153 days, 71 mpox infections were diagnosed.

These infections occurred within a median follow-up period of 85 days from the first dose of vaccine or 86 days in the unvaccinated group. Twenty-one infections were diagnosed in the vaccinated group as compared to 50 in the unvaccinated group

The rate of mpox infection in the vaccinated group was 0.09 for every 1,000 person-days as compared to 0.20 in the unvaccinated group. Thus, the estimated vaccine effectiveness with a single dose of the MVA-BN vaccine was 58%.

There was no meaningful reduction in mpox or bacterial STI rates during the first two weeks from vaccination, thereby confirming that vaccine protection occurred thereafter.

Conclusions

The study findings suggest moderate vaccine effectiveness with one MVA-BN vaccine dose against mpox infection. These results corroborates previous studies on restricted populations with the highest mpox exposure rates conducted in multiple countries.

The small size of the study cohort may have prevented the consideration of confounding factors like smallpox vaccination history, lower access to healthcare, reduced testing because of lower symptomatic status following vaccination. Each of these factorscould have resulted in underdiagnosis, thereby leading to overestimation of vaccine 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.”

Two doses of the MVA-BN vaccine are recommended to contain local mpox outbreaks, especially as most individuals remain unprotected. Targeted interventions aiming at the modification of high-risk sexual behavior, early diagnosis, and prompt isolation, should be combined with vaccination to prevent future mpox outbreaks.