Discovered Asymptomatic monkeypox virus infections among men visiting the largest sexually transmitted infection (STI) clinic in Belgium


Anorectal and oropharyngeal swabs collected for gonorrhoea/chlamydia screening from May 1 until May 31, 2022 were retrospectively tested by a monkeypox-specific PCR. Cases with a positive PCR result were recalled to the clinic for case investigation, repeat testing and contact tracing.

Findings In stored samples from 224 men, we identified three cases with a positive anorectal monkeypox PCR. All three men denied having had any symptoms in the weeks before and after the sample was taken. None of them reported exposure to a diagnosed monkeypox case, nor did any of their contacts develop clinical monkeypox. Follow-up samples were taken 21 to 37 days after the initial sample, by which time the monkeypox-specific PCR was negative, likely as a consequence of spontaneous clearance of the infection.

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We found evidence of asymptomatic MPXV infection in three individuals, in the form of positive PCR results on anorectal samples. Even though the existence of asymptomatic MPXV infection has been suggested by a number of immunological studies in MPXV-exposed individuals,13–19 this was, to our knowledge, never substantiated by direct detection of the virus.

Interestingly, one of the asymptomatic men in our study predates the first detected symptomatic case in Belgium by several days,20 and could not be epidemiologically linked to any other monkeypox case, nor did he report international travel or participation in mass gatherings. This may indicate that MPXV circulated among asymptomatic individuals in Belgium before the outbreak was detected.

Due to the retrospective nature of our study, we were unable to assess whether MPXV could also be detected in other body sites of asymptomatic individuals, such as the respiratory tract. MPXV-PCR cycle threshold values in anorectal samples of the asymptomatic men in this study were similar or lower than those in samples taken from typical monkeypox skin lesions that underwent the same testing procedure in our clinic (mean 24·0 +/-standard deviation (sd) 6·7, n = 52 samples, unpublished data).

This indicates that the anorectal mucosa of asymptomatic cases may be as infectious as skin lesions of symptomatic cases. Similarly, anorectal cycle threshold values of symptomatic cases in our clinic were in the same range (mean 25·5 +/-sd 7·9, n = 43 samples, unpublished data), which presumably indicates similar viral loads. This would support the hypothesis that MPXV can be transmitted via anal sex, even in the absence of symptoms.

Asymptomatic carriership was previously thought to play a negligible role in the spread of orthopoxviruses. Despite the fact that smallpox virus could be detected in the upper respiratory tract of asymptomatic contacts of smallpox cases,21 smallpox eradication was primarily, and successfully, based on the identification and isolation of symptomatic cases.22

Also, during previous monkeypox outbreaks in endemic countries, the role of asymptomatic carriership was never demonstrated. However, it is believed that in monkeypox endemic settings, only a fraction of the true caseload is detected due to a lack of resources to do proper surveillance. Until now, there have been no systematic efforts to detect asymptomatic carriership of MPXV.

It is possible that in the current outbreak in non-endemic settings, asymptomatic carriership plays a more substantial role in virus transmission. In endemic settings, patients typically present with a more generalized rash and transmission occurs mainly within households or health care settings, presumably through direct contact with skin lesions or droplets.2

In the current outbreak, the skin eruption often remains localized at the site of inoculation, and the mode of transmission seems to be sexual.8 In this case, asymptomatic carriership, especially with high viral loads in the anal mucosa, could, therefore, be a significant driver of transmission.

The size of the current outbreak in non-endemic countries is larger than ever documented in the absence of repeated epizootic events. While this may be explained by the high number of close contacts among the affected cases,23 asymptomatic transmission to/from the anorectum in addition to other body sites may have played a role. Indeed, many reported cases so far had unprotected sex with one or several casual sex partners.7,8,24,25

Direct contact with breached anogenital mucosal membranes during sex may be a previously unrecognised mode of MPXV transmission.26 Viral transmission in the absence of noticeable symptoms could explain why self-isolation at symptom onset has been insufficient to halt the epidemic thus far.

We could not confirm MPXV-PCR positivity of the asymptomatic cases in a second sample taken from a different body site or at a different time point. By the time the MPXV-PCR result on the stored DNA extracts was available and the cases returned to the clinic to collect follow-up samples, the infection had likely been cleared. PCR specificity on the original results was supported by consistent PCR amplicon electrophoresis (supplementary figure 1). In addition, 98% of the MPXV genome was retrieved by whole genome sequencing (with an average 161.4x sequencing depth) in the original anorectal sample of case 2 (unpublished data).

It should be noted that the three asymptomatic men in this study may not have been completely free of signs and symptoms of MPXV at the time of initial sampling, as no clinical or anoscopic examination found place at that time, and symptoms may not have been reported because of recall bias or because small and painless perianal lesions went unnoticed.

These findings warrant thorough prospective serological, molecular and epidemiological studies involving monkeypox cases and their contacts. Such studies should elucidate the proportion of asymptomatic cases, to what extent asymptomatic cases are infectious, which body sites contribute to infectiousness, and whether condom use or vaccination could be used to prevent transmission from asymptomatic carriers.

These findings have important consequences for the management of the epidemic. Until the role of asymptomatic transmission is further elucidated, the precautionary principle applies.23 If we want to stop human-to-human transmission, control measures should be revisited. Firstly, awareness campaigns in the general and high risk populations should include the possibility of asymptomatic transmission among close (sexual) contacts.23 Secondly, efforts to identify asymptomatic cases should be increased, by contact tracing, and potentially by screening high risk populations. Finally, our data provide additional evidence to introduce vaccination of high risk populations.


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