Sexual exposures associated with mpox infection: California, November 2022 to June 2023

ABSTRACT Background Exposures associated with mpox infection remain imperfectly understood. Methods We conducted a case-control study enrolling participants who received molecular tests for mpox/orthopoxvirus in California from November 2022 through June 2023. We collected data on behaviors during a 21-day risk period before symptom onset or testing among mpox cases and test-negative controls. Results Thirteen of 54 (24.1%) cases and 5/117 (4.3%) controls reported sexual exposure to individuals they identified as potential mpox cases (“index contacts”; odds ratio [OR]=7.7 [95% confidence interval: 2.5-19.3]). Among these participants, 10/13 (76.9%) cases and 2/5 (40.0%) controls reported their index contacts were not experiencing symptoms visible to participants during sex (OR=14.9 [3.6-101.8]). Only 3/54 cases (5.6%) recalled exposure to symptomatic index contacts. Cases reported greater numbers of anal/vaginal sex partners than controls (adjusted OR=2.2 [1.0-4.8] for 2-3 partners and 3.8 [1.7-8.8] for ≥4 partners). Male cases with penile lesions more commonly reported insertive anal/vaginal sex than those without penile lesions (adjusted OR=9.3 [1.6-54.8]). Cases with anorectal lesions more commonly reported receptive anal sex than cases without anorectal lesions (adjusted OR=14.4 [1.0-207.3]). Conclusions Sexual exposure to contacts known or suspected to have experienced mpox was associated with increased risk of infection, often when index contacts lacked apparent symptoms. Exposure to greater numbers of sex partners, including those whom participants did not identify as index contacts, was associated with increased risk of infection in a site-specific manner. While participants’ assessment of symptoms in partners may be imperfect, these findings suggest individuals without visibly prominent mpox symptoms transmit infection.