Although mpox (previously monkeypox) vaccination reduces disease severity, protection appears to diminish after 2 years, indicating that recent immunization may be more effective, according to study results presented at the Conference on Retroviruses and Opportunistic Infections (CROI) 2026, held from February 22 to 25 in Denver, Colorado.
While previous research suggests that mpox vaccination attenuates disease severity, these studies often lack adjustment for important variables such as age and health status. To help fill this knowledge gap, researchers examined the relationship between mpox vaccination and disease severity with adjustment for potential confounders. They also assessed temporal associations between vaccination date and mpox severity.
In a retrospective analysis, the researchers used surveillance data and medical records from 164 probable and confirmed mpox cases reported in Washington state between January 2023 and June 2025. Approximately 82% of patients were cisgender men and 7% identified as another gender.
The impacts of waning immunity need to be studied further to ensure that communities receive appropriate therapeutics and interventions.
More than half (57%) of patients received at least 1 dose of the Modified Vaccinia Ankara-Bavarian Nordic (MVA-BN) vaccine, while others remained unvaccinated. Among patients in the vaccinated group, the mean (SD) disease severity score based on an adapted version of the Mpox Severity Scoring System was 4.6 (1.7). This was 1.44 (95% CI, 0.79-2.09) points lower than those in the unvaccinated group, indicating reduced severity (P <.001).
Between-group differences remained after adjustment for age and immunocompromising conditions in a linear regression analysis, with a difference of 1.48 (95% CI, 0.77-2.19; P <.001).
Disease severity was also related to time since receipt of last vaccine dose. Severity score was 0.97 (95% CI, 0.07-1.87) points higher among those who were vaccinated more than 2 years before mpox infection compared with vaccination in the year preceding the infection (P =.035). The difference was attenuated and lacked statistical significance, however, after adjustment for age and immunocompromising conditions (0.93; 95% CI, -0.12 to 1.98; P =.082).
The researchers concluded, “Our results suggest that mpox vaccination protects individuals from having more severe disease and that individuals who received their vaccine more than 2 years ago may be at risk of more severe disease than those more recently vaccinated.” They added, “The impacts of waning immunity need to be studied further to ensure that communities receive appropriate therapeutics and interventions.”
References:
Meana B, Fairfortune T, Erly S, Amiya R, Bryan Z, Husain S. Association between mpox vaccination and disease severity in Washington state, 2023-2025. Presented at: CROI 2026; February 22-25, 2026; Denver, CO. Abstract 321.
