Monkeypox may be infectious when asymptomatic, Belgian study finds

Vlad Orlov/

Researchers from the Institute of Tropical Medicine in Antwerp, Belgium, have found three cases from May of men who tested positive for monkeypox in the absence of any symptoms at all.

The viral load seen in the three anorectal swabs is similar to that seen in body fluids from people with symptoms, suggesting that asymptomatic transmission of monkeypox could be taking place.

The study is available as a pre-print but has not been peer reviewed. Dr Irith de Baetselier told a webinar yesterday organised by the European Centre for Disease Control (ECDC) and the European AIDS Clinical Society (EACS) that all three were gay men with HIV who attended the institute’s sexual health clinic for regular chlamydia/gonorrhoea tests. All had had STIs in the past, but none had any other infections this time.

NAM aidsmap's Susan Cole talks to Harun Tulunay, who shares his experience of being hospitalised with severe monkeypox, and BHIVA Chair Dr Laura Waters, about the virus.

As we reported last week, the number of cases and countries affected has continued to grow. Dr Joana Haussig of ECDC reported that they are now aware of 6487 cases in 51 countries worldwide (excluding the African countries where monkeypox has long been endemic). Germany now has the largest number of cases at 1242, with the UK and Spain also having seen over 1000 cases and France, the US and Spain over 400. Asia has seen its first cases with isolated ones in Singapore and South Korea.

Even symptomatic cases are almost certainly under-reported, especially as the most common symptoms are a non-specific rash (75% of cases) or systemic flu-like symptoms (73%). The characteristic lesions appear in 71% of cases but are anogenital in only 59% of them. Another study found that 57% of people with monkeypox also had raised levels of the liver enzyme ALT, which can help in diagnosing people with non-specific symptoms.

The study

To see if there had been any asymptomatic monkeypox infections in patients attending the clinic, the researchers retrospectively tested anorectal and throat samples taken from 224 men between 11 April and 31 May. No anorectal swabs from April, but four from May, tested PCR-positive for monkeypox.

These positive results were then given confirmatory tests using a quantitative DNA assay. These revealed a viral load in anorectal swabs that was similar to or slightly lower than those seen in the body fluids of symptomatic patients.

One of the four patients turned out to have monkeypox symptoms after all: he had complained of a painful perianal rash which had been misdiagnosed as herpes. But the other three all said they had not had any symptoms in the two months prior to testing, or the three weeks afterwards (the average incubation period of monkeypox is 7-8 days but may be as long as 3-4 weeks).

One of the monkeypox-positive samples pre-dates, by several days, the first symptomatic case of monkeypox in Belgium. This case could not be linked to any previously diagnosed monkeypox case, and the person concerned had not travelled abroad or been in any mass gatherings.

This suggests that some asymptomatic spread could have been happening before the appearance of visible symptoms. The authors comment that his might help to explain the rapid expansion of the current epidemic.

The webinar also hear that the three asymptomatic men were recalled 37, 24 and 23 days after their first sample was taken and given a second PCR test, and also tested for antibodies to monkeypox. This time round they were PCR-negative but antibody-positive, showing that they had indeed had monkeypox but that their immune system had cleared it.



Having no symptoms.

polymerase chain reaction (PCR)

A method of amplifying fragments of genetic material so that they can be detected. Some viral load tests are based on this method.


Having symptoms.



Chlamydia is a common sexually transmitted infection, caused by bacteria called Chlamydia trachomatis. Women can get chlamydia in the cervix, rectum, or throat. Men can get chlamydia in the urethra (inside the penis), rectum, or throat. Chlamydia is treated with antibiotics.


A rash is an area of irritated or swollen skin, affecting its colour, appearance, or texture. It may be localised in one part of the body or affect all the skin. Rashes are usually caused by inflammation of the skin, which can have many causes, including an allergic reaction to a medicine.

"During previous monkeypox outbreaks in endemic countries, the role of asymptomatic carriership was never demonstrated," de Baetselier and colleagues write, but there were no systematic efforts to detect it. It is possible that in the current outbreak, "asymptomatic carriership plays a more substantial role in virus transmission”.

The Institute of Tropical Medicine is now offering PCR-testing for monkeypox to all patients who come for gonorrhoea/chlamydia screening, so we may soon know more about asymptomatic infections and the degree to which they contribute to transmission.