Many countries around the world have been affected by the monkeypox (mpox) outbreak since April 2022. The two continents, America and Europe, are among the most affected. The current wave of mpox cases has infected chiefly men who have sex with men (MSM). One of the common symptoms of mpox is anogenital lesions, which indicate transmission of the virus through sexual activities.
Study: Outcomes of post-exposure vaccination by modified vaccinia Ankara to prevent mpox (formerly monkeypox): a retrospective observational study in Lyon, France, June to August 2022. Image Credit: CI Photos / Shutterstock
Health officials have recommended vaccination with the modified vaccinia virus Ankara vaccine (MVA-BN; Bavarian-Nordic) to control mpox transmission. MVA-BN, Bavarian-Nordic, is a third-generation vaccine designed to prevent both smallpox and mpox infections.
Although mpox vaccination has been recommended at both primary vaccinations, i.e., pre-exposure to the virus, and post-exposure preventive vaccination (PEPV), there exists scarce evidence that supports the effectiveness of these two prophylactic strategies. A new Eurosurveillance study investigated the efficacy of the MVA-BN vaccine, which was administered to individuals after exposure to the virus.
About the Study
The French government recommended PEPV with MVA-BN vaccine to ‘at-risk’ contacts on May 20, 2022. At-risk contacts are those individuals who directly (via injured skin and body fluids) or indirectly (via textiles or surfaces) came into contact with the mpox virus. In addition, this group includes those who experienced prolonged exposure, i.e., above 3 hours of exposure to the mpox virus droplet.
The recommended MVA-BN vaccination regime comprises two doses of the vaccine at an interval of greater than or equal to 28 days. The first dose must be provided as early as possible and within 14 days after exposure to mpox. The second dose must be administered 28 days after the first dose. The intervals between the two doses are often changed due to low vaccine availability.
The contact-tracing strategy was used to identify at-risk contacts. Additionally, some individuals self-declared and were available for vaccination. A total of 130 adults received MVA-BN as part of PEPV between June 15 and August 12, 2022. Among these, 108 participants received a single dose of the MVA-BN vaccine within fourteen days of mpox exposure. These participants were included in the current study.
The study cohort consisted of 97 men and 11 women. The median age of the participants was 35 years. 17 participants were found to be immunocompromised. 23% of the cohort had received smallpox vaccination during their childhood.
Breakthrough mpox was defined as the development of symptomatic disease after coming in contact with a mpox patient, within 21 days of exposure, even after receiving PEPV. A breakthrough mpox infection has been considered a failure of the PEPV strategy.
Clinicians confirmed breakthrough infection through diagnostic consultations via real-time PCR on the skin and/or mucosal swabs collected from lesions. The mpox virus was isolated from samples that tested positive for mpox infection by PCR to assess virus infectivity.
Ten percent of the participants developed a breakthrough infection after vaccination. The median time interval between vaccination and symptom onset was five days. However, these patients showed mild symptoms, and none required hospital admission. Analysis of swab samples revealed a median quantification cycle (Cq) value of 29.2.
The finding of this study is in line with another French study that reported 4% of breakthrough mpox infections among the vaccinated contacts. It was observed that people who received the smallpox vaccine were less affected by mpox.
Although initial Cq values of samples from non-vaccinated mpox patients and vaccinated individuals with breakthrough infection were comparable, viral culture in the latter group was observed to turn positive in due course. Therefore, more research is required to validate this finding.
Factors Behind the Failure of the PEPV Strategy
Univariate analysis was used to assess the risk factors associated with breakthrough mpox infection. Sexual exposure was identified as a key factor for breakthrough mpox. Additionally, scientists observed that immunosuppression also aided in breakthrough infection. Interestingly, patients who received mpox vaccination earlier after exposure, i.e., within five days of contact, were more prone to develop a breakthrough infection than those who received the vaccine later.
One of the limitations of the current study is the retrospective design. In addition, all findings were based on a single center and did not have a control group. Owing to the small sample size, the authors failed to draw any conclusions about women or transgender women with mpox infection.
Despite its limitations, the primary importance of the current study is the determination of the efficacy of the PEPV strategy to prevent mpox infection. The study findings indicated that PEPV by a single MVA-BN dose was ineffective in preventing mpox. The main factor responsible for the failure of the PEPV strategy was sexual exposure. Hence, compared to PEPV, pre-exposure vaccination was found to be more effective in breaking sexual transmission chains and containing the mpox epidemic.