Coloured transmission electron micrograph (TEM) of monkeypox virus particles (red) found within an infected cell (blue).

Monkeypox virus particles (red) infect a cell (blue), as shown in this coloured image from a transmission electron microscope.Credit: NIAID/NIH/SPL

As global monkeypox cases continue to soar, researchers are learning more about how the disease is spreading. Early predictions that the virus transmits primarily through repeated skin-to-skin contact between people have largely borne out, according to a tranche of new studies.

“When you put all these studies together, we see that the clinical presentation everywhere is similar — but also surprising,” says Oriol Mitjà, an infectious-disease researcher at Germans Trias i Pujol University Hospital in Barcelona, Spain, who co-authored one of the recent studies in The Lancet1. That’s because the symptoms and pattern of spread don’t look like what researchers had observed in West and Central Africa, where the monkeypox virus has caused isolated, persistent outbreaks for decades.

Since early May, monkeypox has spread to more than 90 countries and led to more than 32,000 infections, with nearly one-third of cases reported in the United States. The virus’s quick spread led the World Health Organization to issue its highest-level public-health alert on 23 July; US President Joe Biden followed suit on 4 August by declaring a US public-health emergency.

Although some women and children have been infected since May, most cases have so far occurred in men who have sex with men (MSM), especially those with multiple sexual partners or who have anonymous sex. The virus has probably been taking advantage of dense sexual networks in the MSM community to spread efficiently, Mitjà says. The more the virus continues to spread, the more opportunities it will have to infect other populations, including wild animals — which scientists have warned might establish viral reservoirs that could infect humans repeatedly.

‘Teeming with virus’

When a person contracts monkeypox, they can develop flu-like symptoms, enlarged lymph nodes and distinctive fluid-filled lesions on their skin. Although some researchers have suggested that the monkeypox virus could spread through respiratory droplets or airborne particles, as SARS-CoV-2 does, Mitjà and his colleagues report that samples from skin lesions, collected at the time of diagnosis, contain much more viral DNA than do those from the throat1. The lesions seem to be comparatively “teeming with virus”, says Boghuma Titanji, an infectious-disease physician at Emory University in Atlanta, Georgia, who was not involved with the study.

Several studies2,3, including Mitjà’s, show that few people contract the disease from an infected household member with whom they didn’t have sexual contact. This finding, paired with the data about viral load, suggests that respiratory droplets and airborne particles probably aren’t the primary transmission route, Titanji says. If corroborated by further research, it could call into question whether people should isolate for the entire duration of infection, which might be difficult because the illness seems to take up to a month to resolve, she adds.

Still missing are detailed data about how a person’s viral load changes over time, says Jessica Justman, an infectious-disease physician at Columbia University in New York City. Although Mitjà and his colleagues didn’t detect much viral DNA in samples that they collected from people’s throats early during infection, it’s possible that if they had collected them later — or even earlier — viral levels could have been higher, she says. Such data, which the team is now collecting in a follow-up study, would allow public-health officials to offer better isolation and treatment guidance to infected people.

Talking about sex

Whether monkeypox is sexually transmitted in absolute terms — passed from one person to another through blood, semen or other bodily fluids during sex — is still unclear. But several studies have found that DNA from the monkeypox virus is present in a person’s semen for weeks after they become infected2,3. One study also isolated infectious virus from a single individual’s semen six days after their symptoms appeared4.

Even if the virus can be sexually transmitted, it’s unclear how large of a role this mode of transmission has, compared with simply being in close, skin-to-skin contact with a person or inhaling their respiratory particles — which also occur during sex. If other studies find infectious virus in semen, understanding how long it can persist in that bodily fluid will be important. Viruses such as Ebola can persist in semen for months, if not years, after infection, which has complicated efforts to prevent outbreaks. Until researchers know more, the UK Health Security Agency recommends that people should continue to use condoms for eight weeks after infection.

Mitjà and his colleagues noticed that, in the people they examined, having a larger number of lesions in the mouth and throat was linked to oral sex, and having more lesions in and around the anus was linked to anal-receptive sex. Given all these findings, Titanji says it’s crucial that public-health officials don’t shy away from talking about sex in their guidance and are explicit about the types of protection available.

More data from rigorously designed studies can’t come fast enough, Justman says. Some researchers already worry that the outbreak is past the point of being contained, given news of inadequate vaccine stockpiles and inaccessible antiviral treatments, as well as insufficient testing. Funding and motivation to study monkeypox are limited compared with COVID-19, she says. “We don’t have an ‘Operation Warp Speed’,” like there was to rev up US vaccine development during the pandemic, she adds.