Asymptomatic and Subclinical Mpox: An Association With Modified Vaccinia Ankara Vaccine.

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 polymerase chain reaction testing on rectal and pharyngeal specimens collected from symptomatic and asymptomatic patients at a sexual health clinic in Seattle, WA, between May 2022 and May 2023. Analyses evaluated the prevalence of asymptomatic or subclinical infection and, among persons with polymerase chain reaction-positive tests, the association of MVA vaccination status with the symptomatic infection. RESULTS:The study population included 1663 persons tested for mpox during 2353 clinic visits. Ninety-three percent of study participants were cisgender men and 96% were men who have sex with men. A total of 198 symptomatic patients (30%) had a first mpox-positive test during 664 visits. Eighteen patients (1.1%) tested during 1689 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 1 dose of the MVA vaccine ( P < 0.0001). Controlling for calendar month, study subjects who received 1 and 2 doses of MVA vaccine were 4.4 (95% confidence interval, 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. Modified Vaccinia Ankara vaccination is associated with an asymptomatic/subclinical infection among persons with mpox.