Until recently, monkeypox rarely spread from person to person. In 2005, a study declared a cluster of six cases in the Republic of Congo “the longest uninterrupted chain of human monkeypox fully documented to date.”
That has changed, to say the least. So far this year, more than 25,000 monkeypox cases have been recorded across 83 countries—and human-to-human transmission is clearly happening at large scale.
How does monkeypox spread among people? Research is ongoing, and findings around monkeypox transmission may develop over time. But here’s what the latest science suggests.
Most cases were once linked to infected animals
The first human case of monkeypox was identified in a baby living in the Democratic Republic of the Congo in 1970, according to the World Health Organization (WHO). For decades after, cases were fairly rare and often linked to contact with infected animals. The U.S. experienced a small outbreak in 2003, with 47 cases in people linked to contact with pet prairie dogs.
Things changed in 2017, when a fairly large human outbreak began in Nigeria. Doctors there diagnosed cases among young men who hadn’t been exposed to infected animals and had lesions on their genitals, as NPR reported. Nigerian researchers published a 2019 study on the outbreak and raised the possibility of sexual transmission, but the theory gained little traction at the time. “There’s a tendency for people to latch [onto] what is tradition, and the tradition is that monkeypox is transmitted from animals to humans,” says study co-author Dr. Dimie Ogoina, president of the Nigeria Infectious Diseases Society.
Most monkeypox cases in the ongoing outbreak are linked to sex
As the current outbreak shows, human-to-human transmission of monkeypox does happen—and sexual activity plays a significant role.
“Right now, most transmission is occurring in queer and gay sexual networks, and most transmission is occurring through sexual or intimate contact,” says Joseph Osmundson, a clinical assistant professor of biology at New York University.
Ogoina says monkeypox primarily spreads among humans through close, skin-to-skin contact—especially with the disease’s hallmark blister-like rash. Researchers are still studying whether people can be asymptomatically contagious, but individuals are considered infectious at least until their rash has fully healed and fresh skin has formed, according to the U.S. Centers for Disease Control and Prevention (CDC).
A study on the current outbreak, published in July in the New England Journal of Medicine, found that, of more than 500 monkeypox cases across 16 countries diagnosed as of June, 95% were linked to sexual activity and 98% were among men who have sex with men. In July, WHO Director-General Tedros Adhanom Ghebreyesus advised men who have sex with men to temporarily reduce their number of partners to minimize their risk.
Is monkeypox a sexually transmitted infection (STI)?
Any form of skin-to-skin contact—not just sexual activity—can potentially spread monkeypox. As a result, monkeypox is not considered a “traditional” STI, according to Dr. Roy Gulick, chief of infectious disease at Weill Cornell Medicine and NewYork-Presbyterian. The CDC also says monkeypox is not considered an STI.
Most cases in the current outbreak have been linked to male sexual activity, but Osmundson says the virus could also spread in settings where there’s lots of close non-sexual contact, such as on sports teams, in spas, or in college dorms.
Can you get monkeypox from surfaces?
It is possible to contract monkeypox through exposure to items, such as clothing or bedding, that have touched an infectious person’s rash. In 2018, a health care worker in the U.K. contracted monkeypox most likely after handling a sick person’s bedding, according to one study.
But there’s little evidence to suggest that “incidental” contact frequently spreads the virus, says Angela Rasmussen, a virologist at the University of Saskatchewan in Canada. “You have to be exposed to enough virus to actually get infected with it,” she says. That might be possible after sleeping in the same bed or sharing a bath towel with an infectious person, but it’s much less likely after fleeting encounters like touching a shared doorknob, she says.
With the vast majority of cases so far linked to sexual activity, it’s important to communicate that there is a “spectrum” of risk associated with monkeypox, says Dr. Müge Çevik, a clinical lecturer in infectious diseases at the University of St. Andrews in Scotland. “Every single exposure [doesn’t carry equal] risk,” she says. “People really need to know where to be vigilant. For example, reducing new [sexual] partners may be more useful than cleaning chairs you’re sitting in in a coffee shop.”
Is monkeypox airborne?
It’s possible to catch monkeypox through exposure to an infectious person’s respiratory fluids, but the WHO says that usually requires close, sustained face-to-face contact. Researchers are still studying how often that type of transmission occurs, according to the CDC.
Scientists have demonstrated that it is possible, under specific experimental conditions, for monkeypox to spread via aerosols, or tiny particles that can stay suspended in the air—but so far, there’s not strong evidence to suggest that’s happening under real-world conditions outside of the lab, Rasmussen says. “This is spreading primarily in communities of men who have sex with men, and that really suggests the primary mode of transmission is direct, prolonged skin-to-skin contact,” she says. If airborne transmission were common, she says, we would likely see far more cases among people from other demographic groups.
In the shadow of COVID-19, people are understandably concerned about monkeypox potentially being transmitted through aerosols. But ”the epidemiology is very different,” Rasmussen says. “These are very different viruses.”
Can kids get monkeypox?
More than 80 children around the world have been sickened by the virus so far, largely as a result of household transmission. A pregnant person can also pass on the virus to their fetus, according to the CDC.
In the 1970s, monkeypox mostly affected young children—but by the 2010s, the virus was more frequently diagnosed among adults, according to a study published in PLOS Neglected Tropical Diseases in February. That’s in part because routine smallpox vaccination (which also protects against monkeypox) ended as smallpox was eradicated, the study’s authors write. Several decades ago, only little kids were young enough to have been born after smallpox vaccination ended. Now, a larger chunk of the population is vulnerable.
Pediatric cases have raised concerns that schools and daycares could become monkeypox hotspots. But Ogoina says that hasn’t happened during Nigeria’s outbreaks, which is a promising sign. “I’m not sure whether it’s something we need to worry about,” he says. “But we need to tread cautiously and look for the evidence.”
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Contributor: Jamie Ducharme