Since monkeypox started making thousands of people sick around the world this spring, two big questions have emerged: Why a virus that has never managed to spread beyond a few cases outside Africa suddenly causes a ‘ such a large and global epidemic? And why do the vast majority of affected men having sex with men (MSM)?
A long history of work on sexually transmitted infections and early studies on the current outbreak suggest the answers may be linked: the virus may have found its way into highly interconnected sexual networks within the MSM community, where it can spread in ways it cannot. 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. It 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 spread, while suggesting that the risk to the wider population remains low.
But there are still many uncertainties and communication is strained due to the risk of stigmatizing MSM and because communicating frankly about sexual behavior is difficult. “I think we need to talk more about sex,” says Gregg Gonsalves, an epidemiologist at the Yale School of Public Health and a former HIV activist. “Everyone was very clear about the stigma and repeated it over and over again. The point is, you still have to face the risk of infection in our community. “
Since the beginning of 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 more common in at least a dozen countries in West and Central Africa, where the virus has animal stocks. More than 60 cases and one death have been confirmed this year.)
The vast majority of cases in the current outbreak were in MSM. Researchers from the UK Health Safety Agency (UKHSA), for example, asked patients to fill out questionnaires. Of the 152 who did, 151 said they were MSM, the team wrote in a technical briefing released on June 10; the remaining patient refused to respond. Other countries have seen similar patterns.
It could be a distorted image, of course. “MSMs have a better relationship with doctors than straight 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 be tested for the virus. “I don’t know if we’re necessarily looking enough into heterosexual social networks to draw the conclusion that this isn’t a broader issue,” says Boghuma Titanji, an Emory University virologist who works at a sexual health clinic.
But most researchers say that such “finding bias” is unlikely to explain the surprising pattern. Although some monkeypox patients have mild infections that can be ignored or misdiagnosed, others have very characteristic rashes and stabbing pains that require hospitalization for pain treatment. If many people outside the MSM community had monkeypox, more would have been shown in the statistics at this point.
Ashleigh Tuite, an infectious disease epidemiologist at the University of Toronto, says she “understands the hesitation” to focus on MSM, given the risk of stigma that could worsen discrimination and cause those affected to delay seeking treatment. “But based on the data we have and based on the contact tracing that has been done, it is very clear that this is an MSM-focused outbreak at this point,” she says. “Anyone can get monkeypox, but we are seeing disease activity mostly among MSMs,” confirms Demetre Daskalakis, an 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 to additional interviews focusing on their sexual health. Among the 45 participants, 44% reported more than 10 sexual partners in the previous 3 months and 44% reported 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 are unsure whether it is important for transmission; skin-to-skin contact may be sufficient. (Other sexually transmitted infections, including herpes and scabies, are also primarily spread this way.)
For those studying how pathogens spread through social and sexual networks, the model is 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 of people was too short to sustain the broadcast. But averages overshadowed the fact that while many people have few sexual partners, some have large numbers. This helps guide transmission because, if infected, they are also more likely to infect others.
Sexual networks between MSMs are no different in nature from those in other groups, Whittles points out, but a core group of people are much more densely connected than people outside the MSM community. They switch partners more frequently and are more likely to have multiple partners at the same time. “These things happen in all sex networks, it’s just a matter of degree,” Whittles says. And in a densely connected network, the virus is less likely to hit a dead end.
“It is entirely possible that this epidemic will spread among a subset of people just because that subset is connected to a network differently from all others,” says Keletso Makofane, a social network epidemiologist at the FXB Center for Health and Human Rights. at Harvard University. Together with colleagues, Makofane hopes to start a study in New York City in August to better understand the spread of the disease. “The idea is to get an idea of how many people report symptoms consistent with monkeypox and how they are related,” he says.
The LSHTM study, published in medRxiv on June 13, used UK data on patterns of sexual union to model the spread of monkeypox among MSMs and beyond that group. Since it is not yet clear 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 prolonged outbreak of more than 10,000 cases among MSM globally is “highly likely,” they write. “In contrast, sustained transmission in the non-MSM population is unlikely in all scenarios considered.”
Because the model is based on UK data, the findings may not apply elsewhere, says lead author Akira Endo. And other factors may have made the outbreak worse. Monkeypox may have mutated in ways that allow it to spread more easily, and the share of the population that has had the smallpox vaccine, which also offers some protection against monkeypox, is decreasing because vaccination against smallpox it has been abandoned all over the world since the 1970s. But the modeling shows “we don’t necessarily need [those factors] to explain the observed patterns, ”says Endo.
Such conclusions put epidemiologists in a delicate position and some have refused to speak Science for fear of stigmatizing MSM. Endo says she understands this and agrees that the results could easily be misunderstood. “In the meantime, I also understand that there is a risk in the other direction, that information won’t reach those who need it most before it’s too late,” she says.
Whittles agrees, calling the results “practical information, in terms of where they are spreading. It’s morally neutral, ”he says. “Knowledge of what is happening is power, even if that knowledge is imperfect and will change,” adds Daskalakis.
The virus may still find other networks with similar characteristics. Daskalakis recalls an outbreak in the United States of methicillin resistant Staphylococcus aureus in the 2000s it started in the MSM community but then spread to gyms, athletes and prisons. Monkeypox may also begin to spread among prostitutes and their clients, Tuite says.
How quickly the virus will spread over the next few months depends on control efforts. National health authorities in Europe, Canada and the United States have published guidelines on how to reduce the risk of infection, and dating apps have warned users of the risk of monkeypox and its symptoms, which can change contact patterns. Increased awareness among healthcare professionals can also have an impact, Whittles says: Faster diagnoses mean patients will isolate themselves earlier in their infection, reducing forward transmission. “So there are a couple of different ways that behavior can change, even if it’s not about people having less sex,” she says. And the model does not take into account infected individuals who develop immunity. This means that “we may see a slowdown in the epidemic sooner than we imagine,” says Endo.
Many countries are also gearing up to launch immunization campaigns. Targeting those who have many sexual partners may be the most efficient approach. In new guidance published on 21 June, UKHSA announced it would begin offering vaccines to higher-risk MSM: “Risk criteria would include a recent history of multiple partners, participation in group sex, participation in sex in clubs. or a proxy marker such as recent bacterial STI (in the past year). “
It’s important to alert that community and get it right, Gonsalves says. “We should say: it’s not about who you are. It’s about what you are doing. And we will not stigmatize it. But just know that you are more at risk if you fit this profile.