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Since the outset of the global monkeypox outbreak in May, public health and infectious disease experts have told the public that the virus is largely transmitting through skin-to-skin contact, in particular during sex between men. 
Now, however, an expanding cadre of experts has come to believe that sex between men itself — both anal as well as oral intercourse — is likely the main driver of global monkeypox transmission. The skin contact that comes with sex, these experts say, is probably much less of a risk factor.
In recent weeks, a growing body of scientific evidence — including a trio of studies published in peer-reviewed journals, as well as reports from national, regional and global health authorities — has suggested that experts may have framed monkeypox’s typical transmission route precisely backward.  
Reconceiving the primary risk factors for transmission is crucial because of how it may affect guidance on reducing the risk of infection, including the question of whether demanding that people with the virus self-isolate has any substantial impact on transmission.
“A growing body of evidence supports that sexual transmission, particularly through seminal fluids, is occurring with the current MPX outbreak,” said Dr. Aniruddha Hazra, medical director of the University of Chicago Sexual Wellness Clinic, referring to monkeypox and to recent studies that found the virus in semen.
Consequently, scientists told NBC News that the Centers for Disease Control and Prevention and other public health authorities should update their monkeypox communication strategies to more strongly emphasize the centrality of intercourse among gay and bisexual men, who comprise nearly all U.S. cases, to the virus’ spread.
On Aug. 14, Dr. Jeffrey Klausner, an infectious disease physician at the University of Southern California, and Dr. Lao-Tzu Allan-Blitz, a resident physician in global health at Brigham and Women’s Hospital in Boston, published an essay on Medium in which they reviewed the science supporting the argument that during the current outbreak, monkeypox is largely transmitting through anal and oral intercourse between men.
“It looks very clear to us that this is an infection that is transmitting sexually the vast majority of the time,” Allan-Blitz said.
This debate, however, is far from settled.
Dr. Rosamund Lewis, technical lead for monkeypox at the World Health Organization, told NBC News it was “unfortunate but true” that “we don’t know yet” whether the virus is predominantly transmitted through intercourse.
“Completely reading the situation as uniquely due to anal or oral sex is highly likely to be overreach,” she said. “The correlation may appear to be strong, but that does not explain the whole picture of disease caused by this virus. So we need to keep an open mind.” 
Some experts in infectious disease see evidence supporting the argument that monkeypox at least transmits more readily through intercourse.
“At this point,” said Dr. Paul Adamson, an infectious disease specialist at the UCLA School of Medicine, “I’m not sure we can say it is primarily the sexual transmission and not the skin-to-skin contact that also occurs during sex that is contributing to the most transmission during this current outbreak. However, emerging data seem to suggest that monkeypox might be more efficiently transmitted sexually.”
In an interview, Klausner, who has submitted a version of his and Allan-Blitz’s essay to a scientific journal for publication, distilled the evidence that he said supports the hypothesis that sex itself fuels the global outbreak into four major points. 
First, he noted that, according to the WHO, more than three quarters of global monkeypox cases are among men 18 to 44 years old. This is a typical age breakdown for diagnoses of sexually transmitted infections among gay and bisexual men, he said. What’s more, in recent studies of pooled monkeypox cases among this demographic, 17% to 32% of those diagnosed with the virus received a sexually transmitted infection (STI) diagnosis at the same time.
Second, during the global outbreak, atypical to what has historically been seen in the 11 African nations where the virus has become endemic since first being identified in humans in 1970, monkeypox lesions have in the majority of cases occurred in men’s genital and anorectal areas. This, experts told NBC News, suggests that these were the sites where the virus first passed into the body.
In a study of 197 monkeypox cases in London men published July 28 in The BMJ, the British Medical Association’s journal, researchers found that 56% had lesions in the genital area and 42% had them in their anorectal regions. And in a study published July 21 in The New England Journal of Medicine, a global team of researchers pooled 538 monkeypox cases — also all in men — from around the world and found that 73% had lesions in the genital or anorectal areas. 
Third, researchers have found monkeypox in semen and have been able to culture that virus, which suggests it could transmit through ejaculation. Also, the authors of two recent studies have detected the virus after taking anal swabs among men who had monkeypox but were asymptomatic, which indicates that the virus might transmit from the anorectal area during anal intercourse before people develop symptoms. Experts say more research is needed on both these fronts.
Referring to bodily fluids such as semen, vaginal fluids and blood, the WHO’s Lewis said, “Research is underway to find out more about whether people can spread monkeypox through the exchange of these fluids during and after symptomatic infection.”
Finally, Klausner noted that scientists have identified an association between specific sexual acts and the location of monkeypox lesions. 
The authors of a paper published Aug. 8 in The Lancet documenting 181 cases of the virus in Spain found that 38% of the men who reported having receptive anal intercourse, called “bottoming,” developed proctitis, or inflammation of the rectum. Just 7% of the men who reported sex with men without bottoming developed this potentially excruciating symptom. Additionally, 95% of the men with tonsillitis reported performing oral sex on a man.
Dr. Oriol Mitjà, an associate professor in infectious disease at the University Hospital Germans Trias i Pujol in Spain and the joint senior co-author of the study in The Lancet, said monkeypox transmits most efficiently when lesions come into contact with mucus membranes in the anorectal area, genitals, mouth and throat.
Monkeypox is more likely to transmit through oral or anal sex than through contact with external skin, which would need some sort of defect, such as a wound, to allow entry of the virus, Mitjà said.
Dr. Dimie Ogoina, a professor of medicine and infectious diseases at Niger Delta University in Nigeria, acknowledged Mitjà’s research supporting the connection between types of sex between men and monkeypox outcomes.
“This is not to say that females or heterosexuals are not at risk of monkeypox or that the female genital mucosa is not prone to abrasions during sexual activity,” Ogoina said.
Some experts, like the WHO’s Lewis, maintain that the main mode of monkeypox transmission remains skin-to-skin contact — including during sex. Others, like Klausner and Adamson, say a number of infectious disease experts may resist believing intercourse is a predominant driver of the current outbreak because that is not how monkeypox has tended to spread in past decades. 
“Historically, the primary mode of transmission of monkeypox was through skin-skin contact, though there might have been some suggestion of sexual transmission in prior outbreaks. It takes some time and additional data to overturn our understanding of transmission,” Adamson said.  
Monkeypox has been diagnosed in 38,019 people in 93 countries during this current global outbreak, according to the CDC. And the WHO reports that among cases with proper data, 97% have been diagnosed in gay, bisexual and other men who have sex with men. The consistency with which cases have remained so overwhelmingly in this demographic, some experts argue, is further evidence that the virus transmits among them through a behavior that is exclusive to the group — anal intercourse and oral sex between men.
Meanwhile, across the global outbreak, the virus is also apparently following the same transmission patterns traditionally seen in Africa. But experts assert that just as in those African nations, when the virus transmits through nonsexual means, it does so with dramatically lower efficiency — and thus at a rate similar to the relatively slow spread seen in Africa.
Specifically, the authors of The New England Journal of Medicine paper estimated that just 0.8% of the cases they analyzed were due to nonsexual close contact and 0.6% were due to household contact. By contrast, 95% of these cases were likely acquired during sex between men. The authors of the Lancet paper estimated that 3% of the cases they analyzed transmitted through nonsexual household contact.
Dr. Monica Gandhi, an infectious disease physician at University of California, San Francisco, said the small number of global monkeypox cases in children have likely been transmitted through cuddling or hugging. She pointed to various STIs, including herpes, that in rare cases can also transmit nonsexually.
“STIs such as syphilis or chancroid are commonly found in children in the tropics, where abrasions on the arms and legs are common,” Mitjà said.
Referring to the recent rapid expansion of the global outbreak, Ogoina said, “It is all about numbers — the more sexual partners, the greater the likelihood for many to become exposed.” 
If monkeypox is indeed overwhelmingly being transmitted through intercourse and rarely through more casual means, this challenges burdensome public health guidelines recommending that people with the virus isolate for the course of their illness, which can last for weeks, Mitjà and his coauthors argued in their paper. 
Klausner called for updated communications from the CDC and other health authorities to emphasize the importance of sexual intercourse to monkeypox’s transmission.
“If we accept that this is how it’s spread, we know how to reduce the spread: by awareness and education and encouraging people for the time being to reduce sex with multiple partners until they get vaccinated,” Klausner said “And if they can’t reduce the behavior, to try to use a condom.”
CDC spokesperson Kristen Nordlund said the agency’s recent analyses “show most diagnosed cases of monkeypox in the United States are associated with sexual and intimate contact, which can involve a range of behaviors. Additional analyses are needed to understand if specific sexual and intimate behaviors that occur during sex are disproportionately contributing to spread.”
Harvard’s Lao-Tzu Allan-Blitz acknowledged the pervasive concern that telling the public that monkeypox transmits sexually among gay men will fuel homophobia. He said there is, however, also a cost to keeping quiet about how the virus apparently transmits: This keeps people at risk from best understanding how to protect themselves. 
“In our silence, we can also do harm,” he said.
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