Retrospective detection of asymptomatic monkeypox virus infections among male sexual health clinic attendees in Belgium

The magnitude of the 2022 multi-country monkeypox virus (MPXV) outbreak has surpassed any preceding outbreak. It is unclear whether asymptomatic or otherwise undiagnosed infections are fuelling this epidemic. In this study, we aimed to assess whether undiagnosed infections occurred among men attending a Belgian sexual health clinic in May 2022. We retrospectively screened 224 samples collected for gonorrhea and chlamydia testing using an MPXV PCR assay and identified MPXV-DNA-positive samples from four men. At the time of sampling, one man had a painful rash, and three men had reported no symptoms. Upon clinical examination 21-37 days later, these three men were free of clinical signs, and they reported not having experienced any symptoms. Serology confirmed MPXV exposure in all three men, and MPXV was cultured from two cases. These findings show that certain cases of monkeypox remain undiagnosed and suggest that testing and quarantining of individuals reporting symptoms may not suffice to contain the outbreak. Findings of unrecognized or asymptomatic monkeypox virus (MPXV) infections with replication-competent virus in humans suggest that a lack of recognized, clinical symptoms could play a role in virus transmission and the magnitude of the 2022 MPXV outbreak.

Automated summary

The magnitude of the 2022 monkeypox virus outbreak has exceeded any previous outbreak, and undiagnosed infections may be driving the epidemic. A study found four men attending a Belgian sexual health clinic to be infected with monkeypox. One man had a painful rash while the other three were asymptomatic, but their infections were confirmed through clinical examination. These findings suggest that undiagnosed cases of monkeypox could contribute to the transmission and scale of the 2022 outbreak.