In a recent study posted to medRxiv* preprint server, researchers estimated the efficacy of a single-dose Modified Vaccinia Ankara-Bavaria Nordic (MVA-BN) vaccine against mpox (MPX) in gay, bisexual, and other men who have sex with men (GBMSM).

The World Health Organization declared the ongoing MPX outbreak a public health emergency of international concern in July 2022. The European Union/European Economic Area (EU/EEA) recorded over 20,000 cases until October 11, 2022. Vaccination using a third-generation smallpox vaccine has been a pivotal component of MPX control. MVA-BN is one such replication-deficient attenuated smallpox vaccine.

It was approved for smallpox and MPX in the United States (US). Studies conducted in Africa suggest that previous-generation smallpox vaccines can protect against MPX. Nevertheless, data on the real-world effectiveness of third-generation smallpox vaccines against MPX were lacking from before the current outbreak. However, preliminary studies from the US and Israel indicate a substantially high level of protection from vaccination against MPX.

About the study

In the present observational study, researchers assessed the effectiveness of a single MVA-BN vaccine dose against symptomatic MPX among GBMSM in England. They implemented a case coverage or screening method comparing vaccination rates among cases with those in the population. Weekly vaccination rates among GBMSM individuals and the estimate of the GBMSM denominator were used to determine coverage.

The authors estimated 89,240 GBMSM individuals were at a high risk of MPX in England. Vaccination sites reported daily vaccinations by dose and cohort. The vaccination status was classified as recent (vaccinated in the current/past week) or one full dose (vaccinated at least two weeks before with a single dose). Second vaccinations were not considered since only fewer individuals received the second dose.

The vaccination status of cases was determined via questionnaires sent to confirmed or probable cases. Cases with an index date of July 4, 2022, or later until October 3, 2022, were included. Cases vaccinated after the index date were included as non-vaccinated. Cases vaccinated before 2022 were deemed non-vaccinated. Those with information only on the vaccination year (2022) without a date were included in the sensitivity analysis.

Findings

Weekly vaccination coverage increased to 47% by early October 2022, with most of the doses administered by August 22, 2022. Overall, 1,102 MPX cases completed questionnaires. Fifty-two cases were excluded from the investigation since they were female or heterosexual males. Of the cases, 460 had an index date of July 4, 2022, or later. Ninety-seven cases lacked an exact date of vaccination and were excluded; thus, 363 cases were considered for analysis.

Among these, eight cases were vaccinated more than two weeks before rash onset, 32 were vaccinated in less than two weeks, and the remaining were non-vaccinated, including those vaccinated many years before 2022. None of the cases reported second vaccination. The return rate of questionnaires was 33%.

The effectiveness of a single smallpox vaccine dose was 78%. In the sensitivity analyses, reducing the GBMSM denominator by 20%, which increased population vaccination coverage, resulted in a higher VE of 85%. Likewise, increasing the GBMSM denominator by 20% reduced effectiveness to 71%. Vaccine effectiveness was -4% within two weeks of receiving the single dose. When considering cases aged 50 or lower, the effectiveness was 74%.

Conclusions

The researchers estimated the effectiveness of one dose of the MVA-BN vaccine against symptomatic MPX to be 78% after a minimum of two weeks post-vaccination. There was no evidence of protection against disease in the 13 days post-vaccination. These estimates were concordant with the Israeli study, which found 79% efficacy of single-dose MVA-BN vaccine in GBMSM individuals.

The study’s limitations include the assumption that all cases were GBMSM unless they identified explicitly as heterosexual or female. Further, the researchers used questionnaire data for vaccination status since a national vaccination registry was unavailable. Moreover, people vaccinated pre-2022 were considered non-vaccinated, and if prior vaccination conferred protection to some degree, these estimates might have been lower than the actual effectiveness.

To summarize, the findings indicated that a single dose of the MVA-BN offered a relatively high level of protection against MPX. This suggested that when at-risk individuals exceed the vaccine supply for the two-dose regimen, prioritizing the administration of a single dose may be beneficial. Furthermore, future investigations are warranted to examine the durability of protection and effectiveness after two doses.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Tarun Sai Lomte

Tarun is a writer based in Hyderabad, India. He has a Master’s degree in Biotechnology from the University of Hyderabad and is enthusiastic about scientific research. He enjoys reading research papers and literature reviews and is passionate about writing.