Why the monkeypox epidemic mainly affects men who have sex with men

Since monkeypox began sickening thousands of people around the world this spring, two big questions have arisen: why a virus that has never managed to spread beyond a few cases outside of Africa is causing there suddenly such a large global epidemic? And why are the overwhelming majority of men who have sex with men (MSM) affected?

A long history of work on sexually transmitted infections and early studies of the current epidemic suggest that the answers may be linked: the virus may have made its way into highly interconnected sexual networks within the MSM community, where it can spread in ways that it cannot in the general population.

An epidemiological modeling study published as a preprint last week by researchers at the London School of Hygiene & Tropical Medicine (LSHTM) supports this idea. This suggests that the epidemic will continue to grow rapidly if the spread is not reduced. It also has implications for how to protect those most at risk and limit the spread, while suggesting that the risk to the general population remains low.

But there are still many uncertainties and communication is strained because of the risk of stigmatization of MSM and because it is difficult to communicate frankly about sexual behavior. “I think we need to talk more about sex,” says Gregg Gonsalves, a Yale School of Public Health epidemiologist and former HIV activist. “Everyone has been very clear about the stigma and repeats it over and over again. The fact is you still have to deal with the risk of infection in our community.”

Since early May, more than 2,000 cases of monkeypox have been reported in more than 30 countries where the virus is not normally seen. (Outbreaks are most common in at least a dozen West and Central African countries where the virus has animal reservoirs. More than 60 cases and one death have been confirmed there this year.)

The vast majority of cases in the current outbreak have been among MSM. Researchers from the UK Health Security Agency (UKHSA), for example, asked patients to complete questionnaires. Of 152 who did, 151 said they were MSM, the team wrote in a technical brief published June 10; the remaining patient declined to respond. Other countries have experienced similar patterns.

This could be a biased picture, of course. “MSM have a better relationship with doctors than heterosexual men,” says Lilith Whittles, an infectious disease modeler at Imperial College London, which could mean they are more likely to report symptoms of monkeypox and getting tested for the virus. “I don’t know if we search heterosexual social media enough to conclude that this isn’t a larger problem,” says Boghuma Titanji, a virologist at Emory University who works in a sexual health clinic. .

But most researchers say such a “finding bias” is unlikely to explain the striking pattern. Although some monkeypox patients have mild infections that can be missed or misdiagnosed, others have very characteristic rashes and excruciating pain that require hospitalization for pain treatment. If many people outside the MSM community had monkeypox, more would have appeared in the statistics by now.

Ashleigh Tuite, an infectious disease epidemiologist at the University of Toronto, says she “understands the reluctance” to focus on MSM, given the risk of stigma that could deepen discrimination and induce those concerned to delay seeking care. “But based on the data that we have and based on the contact tracing that has been done, it’s very clear that at this stage this is an epidemic focused on MSM,” she says. . “Anyone can get monkeypox, but we’re seeing disease activity mostly in ‘MSM,’ confirms Demetre Daskalakis, HIV prevention specialist at the US Centers for Disease Control and Prevention.

Sexual encounters clearly play a role in transmission. Of the 152 people in the UKHSA dataset, 82 were invited for additional interviews focusing on their sexual health. Among the 45 participants, 44% said they had more than 10 sexual partners in the previous 3 months and 44% said they had group sex during the incubation period. Exactly how the virus is transmitted is less clear. Researchers have found viral DNA, and even infectious viruses, in the semen of some patients, but they’re not sure if that’s important for transmission. skin-to-skin contact may be sufficient. (Other sexually transmitted infections, including herpes and scabies, are also spread primarily this way.)

For those who study how pathogens spread through social and sexual networks, the pattern comes as no big surprise. In the 1970s and 1980s, researchers trying to understand the spread of sexually transmitted diseases such as gonorrhea were perplexed, says LSHTM epidemiologist Adam Kucharski: Survey data showed that the average number of sexual partners was too low to maintain transmission. But the averages hide the fact that while many people have few sexual partners, some have many. This helps to promote transmission because, if they are infected, they are also more likely to infect others.

Sexual networks among MSM are no different in nature from those of other groups, Whittles points out, but a core group of people are much more densely connected than people outside the MSM community. They change partners more frequently and are more likely to have multiple partners at the same time. “These things happen in all sexual networks, it’s just a matter of degree,” says Whittles. And in a densely connected network, the virus is less likely to find itself in a dead end.

“It’s entirely possible that this epidemic is raging among a subset of people simply because that subset is connected in a network differently than everyone else,” says Keletso Makofane, social media epidemiologist at the FXB Center for health and human rights at Harvard University. Together with his colleagues, Makofane hopes to launch a study in New York in August to better understand the spread of the disease. “The idea is to get an idea of ​​how many people are reporting symptoms consistent with monkeypox and how they are related,” he says.

The LSHTM study, published on medRxiv on June 13, used UK data on sexual partnership patterns to model the spread of monkeypox among MSM and outside this group. As it is not yet known how contagious the virus is, the researchers modeled scenarios based on different levels of risk. Without effective intervention measures or behavioral changes, a large and sustained epidemic with more than 10,000 cases among MSM worldwide is “very likely”, they write. “In contrast, sustained transmission in the non-MSM population is unlikely under all scenarios considered.”

Because the model is based on UK data, the results may not apply elsewhere in the world, says first author Akira Endo. And other factors may have worsened the outbreak. Monkeypox may have mutated in a way that allows it to be transmitted more easily, and the share of the population that has received the smallpox vaccine – which also offers some protection against monkeypox – is declining because vaccination against smallpox was abandoned worldwide from the 1970s. But modeling shows that “we don’t necessarily need [those factors] to explain the observed patterns,” says Endo.

Graeme Lamb runs a bathhouse for MSM in Toronto that recently hosted a monkeypox “vaccination clinic” for people at high risk of infection. Steve Russell/Toronto Star via Getty Images

Such findings put epidemiologists in an awkward position, and some refuse to talk to Science for fear of stigmatizing MSM. Endo says he understands that and agrees the results could easily be misunderstood. “In the meantime, I also understand there is a risk the other way around – that the information doesn’t reach those who need it most until it’s too late,” he says.

Whittles agrees, calling the results “practical information, in terms of dissemination. It’s a morally neutral thing,” she says. “Knowledge of what is happening is power, even if that knowledge is imperfect and will change,” Daskalakis adds.

The virus could still find other networks with similar characteristics. Daskalakis recalls an American epidemic of methicillin resistants Staphylococcus aureus in the 2000s that started in the MSM community but then spread to gyms, among athletes, and in prisons. Monkeypox could also start spreading among sex workers and their clients, says Tuite.

How quickly the virus spreads in the coming months will depend on control efforts. National health authorities in Europe, Canada and the United States have issued advice on how to reduce the risk of infection, and dating apps have alerted users to the risk of monkeypox and its symptoms, which can edit contact patterns. Increased awareness among healthcare workers can also have an impact, says Whittles: Faster diagnoses mean patients will isolate themselves earlier in their infection, which will reduce onward transmission. “So there are many ways behavior can change, even if it’s not people having less sex,” she says. And the the model did not account for infected individuals developing immunity. This means that “we could see a slowdown in the epidemic sooner than we imagine,” says Endo.

Many countries are also preparing to launch vaccination campaigns. Vaccination of people with multiple sexual partners may be the most effective approach; in the Canadian province of Quebec, the vaccine is already offered to MSM who have had two or more sexual partners in the past 14 days. It’s important to alert this community and do it the right way, Gonsalves says. “We should say: it’s not about who you are. It’s about what you do. And we are not going to stigmatize it. But just know that you are more at risk if you fit this profile.

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