monkeypox vaccine
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A registered nurse administers an intradermal monkeypox vaccine shot at a vaccination site in Miami on Monday.
The claim that skin-to-skin contact during sex between men, not intercourse itself, drives most monkeypox transmission is likely backward, a growing group of experts say.

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 — includinga trioof studies published in peer-reviewed journals, as well as reports fromnational, regionaland 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 monkeypoxcommunication 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."

Parsing the evidence

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 havefound monkeypox in semen and have been able to culture that virus, which suggests it could transmit through ejaculation. Also, the authors of tworecent 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.