Asymptomatic and Subclinical Mpox: An Association with Modified Vaccinia Ankara (MVA) Vaccine

Abstract

Abstract Background How often mpox causes asymptomatic infections, particularly among persons who have received the Modified Vaccinia Ankara (MVA) vaccine, is unknown. Methods We performed mpox PCR testing on rectal and pharyngeal specimens collected from symptomatic and asymptomatic patients at a sexual health clinic in Seattle, WA, USA between May 2022 and May 2023. Analyses evaluated the prevalence of asymptomatic or subclinical infection and, among persons with PCR positive tests, the association of MVA vaccination status with the symptomatic infection. Results The study population included 1663 persons tested for mpox during 2,353 clinic visits. Ninety-three percent of study participants were cis-gender men and 96% were men who have sex with men. A total of 198 (30%) symptomatic patients had a first mpox positive test during 664 visits. Eighteen (1.1%) patients tested during 1,689 visits had asymptomatic or subclinical mpox based on a positive rectal or pharyngeal test done in the absence of testing done because of clinical suspicion for mpox. Fourteen (78%) of 18 persons with asymptomatic/subclinical mpox and 53 (26%) of 198 persons with symptomatic mpox had received at least one dose of the MVA vaccine (p < .0001). Controlling for calendar month, study subjects who received one and two doses of MVA vaccine were 4.4 (95% CI 1.3-15) and 11.9 (3.6-40) times more likely to have asymptomatic versus symptomatic mpox, respectively, than persons who were unvaccinated. Conclusions Asymptomatic mpox is uncommon. MVA vaccination is associated with an asymptomatic/subclinical infection among persons with mpox.