Monkeypox Virus Seroprevalence among Adults Assessed at Roadside Wellness Centers along Major Transportation Corridor, Kenya, 2025

Background: Since July 2024, Kenya has been experiencing its first monkeypox outbreak. Although over 1,000 cases have been confirmed, gaps in case‑based surveillance could lead to under-ascertainment. To better quantify unrecognized transmission, we conducted a monkeypox serosurvey at clinics along a high‑mobility transportation corridor in Kenya. Methods: We performed a cross-sectional serosurvey at eight clinics between 29 September to 28 October 2025. Adults ≥18 years completed a standardized questionnaire and provided blood samples tested using a multiplex‑bead serologic assay capable of distinguishing likely infection from orthopoxvirus vaccination. Seroprevalence was defined as the proportion of participants who tested seropositive out of all participants with valid test results. Associations with likely infection were assessed using modified Poisson regression with cluster‑robust standard errors. To assess under‑ascertainment, we compared the number of participants with serologic evidence of likely monkeypox infection to the number self‑reporting a prior laboratory‑confirmed monkeypox case. Findings: Of 2,896 participants with valid results, 332 (11.5%) were seropositive, including 82 (2.8%) classified as likely infection. Sex workers (6.4%) and truck drivers (4.1%) had the highest likely infection prevalence. Four participants reported prior confirmed monkeypox cases; all were seropositive. The ratio of persons with likely infections to previously confirmed cases was 21:1. Interpretation: These findings indicate under‑ascertainment of monkeypox infections in high‑risk communities in Kenya. Despite some evidence of undetected transmission, most participants lacked serological evidence of prior orthopoxvirus exposure indicating a potentially large susceptible population. These findings suggest diagnostic access, vaccination among high-risk groups, and ongoing serosurveillance may be important considerations for strengthening outbreak response.