Clinically inapparent mpox virus (MPXV) infections among clients of three anonymous Community Based Voluntary...

Since the mpox outbreak in 2022, it was unclear if and how often infections with mpox virus (MPXV) were clinically inapparent, i.e. not presenting to clinical care with mpox symptoms. Moreover, it was hypothesized that MPXV circulated in the affected communities before the outbreak was officially detected.

We retrospectively tested rectal and urethral swabs, and pooled samples for presence of MPXV. Samples were obtained from routine STI testing of three anonymous Community Based Voluntary Counselling and Testing (CBVCT) centres in Berlin, in 2022 and 2023. Testing results were linked to anonymously provided behavioural data.

Overall, 9,053 samples from 6,600 client visits were included. Clinically inapparent MPXV infections were detectable in 1.1% of the samples. We did not find MPXV infections in the month before the first cases appeared in Berlin or between October 2022 and January 2023 when case numbers were low in Germany. However, during the outbreak period in 2022, we found clinically inapparent MPXV infections among 2.2% of the clients and during summer/autumn 2023 among 0.3%. The number of condomless anal/vaginal intercourse partners within the previous 6 months and PrEP use were identified as predictors of clinically inapparent MPXV infection.

Clinically inapparent MPXV infections occurred during the mpox outbreak in Berlin in 2022 and post-outbreak in summer/autumn 2023. Unrecognized MPXV circulation in Berlin before the recognition of the outbreak in May 2022 appears unlikely. However, low-level sustained circulation of clinically inapparent MPXV infections need to be acknowledged in mpox prevention strategies.