Mpox virus particles (colorized electron microscopic image) National Institute of Allergy and Infectious Diseases (NIAID). Credit: National Institute of Allergy and Infectious Diseases (NIAID)
Universitätsmedizin Berlin has found that a single dose of the Imvanex vaccine provides protection against mpox with 84% effectiveness. For people with HIV, however, a single dose of the vaccine fails to offer sufficient protection. All at-risk groups, and people with HIV in particular, should therefore receive the second dose of the vaccine as recommended. The results of the study have now been published in The Lancet Infectious Diseases.
Since a wave of infections with the mpox virus swept around the world in 2022, Germany’s Standing Commission on Vaccination (STIKO) has recommended that people at an elevated risk of mpox infection receive an Imvanex vaccination. Originally developed to protect against smallpox, the European Medicines Agency (EMA) approved Imvanex for use to protect against mpox in July 2022 in response to the public health emergency.
The mpox virus is related to the original smallpox virus (Variola major). The EMA’s decision to approve Imvanex was based on laboratory data showing that the vaccine offers cross-protection. Until now, however, the extent of this protective effect was not clear, especially for at-risk groups.
Comprehensive study with over 9,300 participants
The EMA commissioned a team led by Prof. Leif Erik Sander, Director of the Department of Infectious Diseases and Critical Care Medicine at Charité and a research group leader at the Berlin Institute of Health at Charité (BIH), to conduct a comprehensive study into the vaccine’s effectiveness against mpox. This study compared effectiveness in people with and without HIV for the first time.
“Our results confirm that a single dose of the vaccine provides good protection against mpox, at least for a short time,” says Sander. “However, this only applies to people not living with HIV. Unfortunately, we found that people with HIV—even those taking adequate medication—are not sufficiently protected by a single dose.”
Conducted between July 2022 and December 2023, the study examined over 9,300 men and trans people who reported having sex with changing men or trans people. These are among the groups for which STIKO recommends vaccination. Half of the study participants received a single dose of the Imvanex vaccine, while the other half remained unvaccinated. Both groups were monitored for two months on average to determine how many participants contracted an mpox infection.
One dose protects those with a healthy immune system
Far fewer mpox cases were recorded for HIV-negative participants in the vaccinated group than in the unvaccinated group, with the vaccine achieving 84% effectiveness. “That is a very good figure, which is likely increased even further by the second vaccine dose,” says Sander. However, due to the significant drop in infections in the second half of 2022, the study was unable to determine the additional effect of a second vaccine dose.
By contrast, for people living with HIV, the study only identified a small, statistically insignificant protective effect. “The reason is presumably that developing immune protection after vaccination requires specific immune cells called T cells,” explains the infectious disease expert.
“These T cells often appear at lower levels in people with HIV and are not fully functional, which translates to a weaker immune response. This also corresponds to our observation that these participants experienced fewer local and systemic side effects after receiving the vaccine.”
Two vaccine doses recommended for all, especially for people with HIV
“We assume that people living with HIV develop protection against mpox after the second vaccine dose, and urgently advise these people to receive the two vaccine doses recommended by the STIKO,” emphasizes Prof. Florian Kurth. Head of the Clinical Infection Research Group at Charité, Kurth played a leading role in the study alongside Sander.
“We recommend that people in all other at-risk groups also complete the two-dose regimen. The immune system typically develops longer-lasting immune protection when exposed to the vaccine on more than one occasion.” Further studies will be required to determine the precise extent of the protective effect in different groups following two vaccine doses.
The research team observed that vaccinated participants who still contracted the virus experienced milder symptoms. They developed fewer pox lesions, which also healed more quickly, and were less likely to report signs of systemic disease such as fever.
“We assume that the second vaccination further reduces the manifestation of such symptoms,” says Kurth. “Fewer pox lesions presumably also reduces the risk of transmitting the virus. Full vaccination should therefore ward off mpox outbreaks.”
Subjects tolerate vaccine well
The researchers also examined the tolerability and safety of the mpox vaccine in over 6,500 people. The most common reaction reported was pain around the injection site. Less than 3% of vaccinated participants reported more pronounced symptoms, such as fever, headache, muscle pain, nausea or diarrhea.
“The mpox vaccine is, therefore, safe and well tolerated overall,” summarized Kurth. “It is important to note that immune protection develops fully around 14 days after vaccination. In addition, people should take general preventive measures, such as using condoms—including to protect against other sexually transmitted diseases.”
The results on vaccine protection relate to clade IIb of the mpox virus, which circulated in Germany during the study period. However, clade IIb is closely related to clade I, which is currently rampant in Central Africa and neighboring regions, so the researchers expect a very high level of cross-protection.
Consequently, the results of this study could also be relevant to the current clade I outbreak in Africa. It remains unclear how long the protection from the vaccine lasts. In the next stage, the research team plans to conduct long-term studies and investigate the effects of a third vaccine dose.
The study was divided into two study arms to examine the tolerability and effectiveness of the mpox vaccine. In the study arm examining safety and tolerability, researchers prospectively examined and regularly surveyed roughly 6,500 people. In the study arm examining effectiveness, researchers applied a rolling cohort design in an emulated target trial. Retrospective comparison of data on over 9,300 vaccinated and unvaccinated subjects with comparable demographic and clinical characteristics simulated a randomized clinical trial. Across all participants—with and without HIV—the average effectiveness of the vaccine stood at 58%.
More information:
Angelo Roberto Raccagni et al, Mpox vaccination: a dose of protection, but is it enough?, The Lancet Infectious Diseases (2025). DOI: 10.1016/S1473-3099(25)00082-9
Provided by
Charité – Universitätsmedizin Berlin
Citation:
Single vaccine dose shows 84% effectiveness against mpox, but insufficient for HIV patients (2025, March 19)
retrieved 19 March 2025
from https://medicalxpress.com/news/2025-03-vaccine-dose-effectiveness-mpox-insufficient.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.
