Efficacy of a single dose of MVA-BN smallpox vaccine against Mpox

Many parts of the world experienced Mpox outbreaks in 2022, including the UK, Europe, and North America. On July 23, 2022, the World Health Organization announced this outbreak as a Public Health Emergency of International Concern. This disease was primarily found among gay, bisexual, and other men who have sex with men (GBMSM). Therefore, vaccination is crucial in containing the outbreak and protecting the vulnerable group from contracting Mpox.

Study: Effectiveness of one dose of MVA–BN smallpox vaccine against mpox in England using the case-coverage method: an observational study. Image Credit: Dotted Yeti / Shutterstock

Background

By mid-2022, more than 20,000 Mpox cases were recorded in the European Union and European Economic Area countries, along with four deaths. Around 3,500 Mpox cases were reported in the UK. A third-generation smallpox vaccine, i.e., Modified Vaccinia Ankara–Bavaria Nordic (MVA–BN), played an important role in containing the outbreak. However, not much data is available regarding the clinical efficacy of this vaccine against Mpox in humans.

The UK Health Security Agency (UKHSA) formulated strategies to control the Mpox outbreak in the UK, including isolation of infected persons and those in close contact, raising awareness among susceptible groups and healthcare professionals, and surveillance of contacts. 

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UKHSA also recommended post-exposure vaccination for those in close contact with individuals infected with Mpox, such as healthcare workers and laboratory staff. Healthcare workers were also advised for pre-exposure vaccination who were at a high risk of being in contact with Mpox samples or patients with Mpox.

Owing to an increasing number of Mpox cases, the Joint Committee on Vaccination and UKHSA recommended pre-exposure Mpox vaccination for GBMSM. As a result, the healthcare officials identified eligible persons and started a vaccination program in June 2022. 

In 2019, the US Food and Drug Administration (FDA) approved MVA–BN for smallpox and Mpox prevention. This vaccine received approval based on immunogenicity studies. Two doses of the vaccine, with 28 days interval between each dose, exhibited the generation of substantial levels of antibodies required to protect individuals from smallpox. These antibodies were considered to confer rapid protection against Mpox as well.

Previous studies in Africa provided evidence that antibodies generated after smallpox vaccination can also confer protection against Mpox. However, there is no real-world data related to MVA–BN vaccines' effectiveness against Mpox disease in humans. 

Interestingly, a relatively recent study conducted in the Netherlands revealed that vaccinated individuals possessed low levels of neutralizing antibody titers against the Mpox virus after one dose, and no increase in antibody levels was observed after the second dose of the vaccine.

About the Study

A recent The Lancet Infectious Disease study focussed on estimating the effectiveness of a single dose of MVA-BN against Mpox in England. This observational study was conducted following the large-scale vaccination program that immunized the GBMSM group with the MVA-BN vaccine.

Vaccine effectiveness was assessed against the laboratory-confirmed Mpox cases in the study cohort using the case-coverage method. The vaccination status of the participants was classified as recent, i.e., those vaccinated within the previous week, and one full dose, i.e., individuals vaccinated with the first vaccine dose more than two weeks ago. The MVA-BN vaccine was primarily delivered subcutaneously.

Study Findings

A total of 1,102 cases had responded to questionnaires, among which females or self-declared heterosexuals were excluded from the study. The MVA–BN vaccine effectiveness against symptomatic Mpox infection, at least 14 days after the first dose of vaccine, was estimated to be 78%. 

This finding was consistent with another study conducted in Israel that reported 79% effectiveness of a single dose of MVA–BN against Mpox in a similar GBMSM cohort. Nevertheless, this study did not estimate the vaccine effectiveness after 14 days, with a follow-up of 25 days after vaccination. A US-based study conducted in a similar time frame revealed a significant increase in Mpox cases among unvaccinated individuals compared to those who received at least one dose of the vaccine.

Notably, the vaccine effectiveness rate estimated after a single dose was found to be consistent with preclinical studies related to immunogenicity responses in animal models. It must be noted that there is a high possibility that vaccine effectiveness varies across populations. Not much information is available regarding the risk factors that influence vaccine outcomes. Nevertheless, this study documented that in four out of eight Mpox breakthrough cases, the patients were HIV positive.

Study Limitations

The current study has several limitations, including the assumption that all cases considered in this study were vaccine-eligible GBMSM unless they were identified as female or heterosexual. Therefore, there is the possibility that a small number of participants were not eligible for vaccination. Since there were some uncertainties about the GBMSM denominator, the resulting vaccine effectiveness ranged between 75% and 87% in the sensitivity analyses. Another limitation of the current study is the inability to adjust potential confounders due to the aggregated form of vaccine coverage data.

Conclusions

Despite the limitations, the current study indicated a relatively high level of protection for the young adult GBMSM population after a single MVA–BN vaccine dose against symptomatic Mpox infection. In the future, researchers must determine the duration of protection and vaccine effectiveness after a two-dose vaccination regime.

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.