A single dose of MVA-BN vaccine provides 58% protection against ampox

The study confirms the efficacy of the single-dose MVA-BN vaccine in reducing ampox infection in high-risk populations, and highlights the need for expanded access and targeted interventions.

Study: Efficacy of the modified vaccinia Ankara-Bavarian Nordic vaccine against ampox infection: simulation of a target trial. Image courtesy: Marina Demidyk / Shutterstock.com

A study recently published BMJ The efficacy of the modified vaccinia Ankara-Bavarian Nordic (MVA-BN) vaccine against ampox has been confirmed.

Global efforts to control ampox

Ampox was diagnosed in more than 20 countries for the first time in May 2022, leading the World Health Organization (WHO) to declare it a public health emergency of international concern on July 23, 2022. The WHO recommends the use of second- or third-generation vaccines against smallpox to protect high-risk individuals from ampox infection.

MVA-BN is a third-generation live attenuated smallpox vaccine that contains non-replicating virus. The vaccine was initially rolled out in June 2022 in Ontario, Canada as post-exposure prophylaxis to protect primarily men who have sex with men (MSM) and sex workers from Mpox infection.

Due to limited vaccine supply, only a single vaccine dose was administered, with the second dose recommended 28 days after the first. By September 30, 2022, a two-dose schedule was introduced.

Observational data on the effectiveness of MVA-BN against ampox infection have indicated vaccine effectiveness ranging 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 groups to simulate a trial setting. The study included 9,803 men aged 18 years and older who were tested for syphilis in the past year.

All study participants had a diagnosis of at least one bacterial sexually transmitted infection (STI) in the same period, 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 given the subcutaneous MVA-BN vaccine between June 12, 2022, and October 27, 2022. Rates of laboratory-confirmed Mpox infection were then compared between the study group and unvaccinated men 15 days after vaccination.

The study period began on June 12, 2022, corresponding with the earliest availability of pre-exposure vaccination. By comparison, the outbreak began to subside on October 27, 2022, at which time 691 individuals in Ontario had been diagnosed with ampox.

The two groups were matched on age, location, HIV history, frequency of bacterial STIs in the past 3 years, and any other AmPox vaccine received in the past year.

Vaccine effectiveness

A total of 3,204 men were included in both vaccinated and unvaccinated groups. 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 the vaccine or 86 days in the unvaccinated group. Twenty-one infections were diagnosed in the vaccinated group, while 50 infections were diagnosed in the unvaccinated group

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

There was no significant reduction in AmPox or bacterial STI rates during the first two weeks after vaccination, confirming that vaccination provided protection thereafter.

conclusion

The study findings suggest moderate vaccine effectiveness with a single MVA-BN vaccine dose against ampox infection. These results confirm previous studies conducted on restricted populations with the highest ampox exposure rates in several countries.

Confounding factors such as history of smallpox vaccination, poor access to healthcare, and fewer tests due to less symptomatic status after vaccination could not be considered due to the small size of the study group. Each of these factors may lead to underdiagnosis, leading to overestimation of vaccine effectiveness.

In the absence of randomised clinical trials, our findings strengthen the evidence that MVA-BN is effective in preventing ampox infection and should be made available and accessible to at-risk communities.,

Two doses of MVA-BN vaccine are recommended to prevent local ampox outbreaks, especially when most individuals remain unprotected. Targeted interventions aimed at modification of high-risk sexual behaviour, early diagnosis and prompt isolation should be combined with vaccination to prevent future ampox outbreaks.