Monkeypox DNA detected in anal samples of asymptomatic men in Paris and Belgium
Simon Collins, HIV i-Base
Testing from sexual health screening in Paris from 5 June to 11 July 2022, reported high levels of monkeypox (MPX) in participants with symptoms but also identified MPX in anal samples of men who were asymptomatic.
The cohort included 706 men who were either HIV positive and on ART or HIV negative and on PrEP. Out of 383/706 men who had symptoms suggestive of MPX (40% had anal lesions), MPX infection was confirmed in 271/383 cases.
Anal swabs for gonorrhoea and chlamydia were analysed in men without MPX symptoms and only those that were negative were then tested retrospectively for MPX. Of these, 13/200 samples (6.5%) were PCR-positive for MPX, despite the men having no symptoms; 8/13 were HIV positive, all on ART with undetectable viral load.
All were advised to not have sex for the next three weeks and 2/13 later developed symptoms of MPX.
Although PCR-positive results do not show whether this represents infectious MPX, it indicates cases of asymptomatic infections that are not currently being recorded.
Positive monkeypox PCR results were also reported in 3/244 retrospectively-tested from anal swabs from a sexual health clinic in Belgium. Follow-up samples tested negative after 21 to 37 days. 
However, in 2/3 cases, the virus isolated from the swabs was replication competent, adding to evidence that onward transmission could be possible during asymptomatic MPX.
This early concern is also discussed in a new commentary article in JAMA. This review suggests asymptomatic transmission may be underestimated, not least because by the time someone has symptoms, they are unlikely to want to spend time in social situations where MPX is easily spread. 
- Ferré VM et al Detection of monkeypox virus in anorectal swabs from asymptomatic men who have sex with men in a sexually transmitted infection screening program in Paris, France. Annals Internal Medicine, Letter. DOI: 10.7326/M22-2183. (16 August 2022).
- De Baetselier P et al Asymptomatic monkeypox virus infections among male sexual health clinic attendees in Belgium. MedRxiv preprint. doi: 10.1101/2022.07.04.22277226.
- Abbasi J. Reports of Asymptomatic Monkeypox Suggest That, at the Very Least, Some Infections Go Unnoticed. JAMA. doi:10.1001/jama.2022.15426. (31 August 2022).