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Just a few months ago, it looked like the U.S. had lost its chance to eliminate the spread of monkeypox – that is, stamp out the outbreak and get cases down to zero, except for new infections that come from abroad.

Experts worried it was just a matter of time before the virus started spreading more widely in the U.S., especially in settings like daycare centers and college dorms.

Now it's clear those concerns did not materialize. Some infectious disease experts are even raising the idea that the U.S. could eliminate the virus.

Monkeypox cases have declined since a peak in early August – from 440 cases a day, down to 60 – and they're the lowest they've been since June. The virus has continued to circulate almost entirely within gay and queer sexual networks. And vaccine supply is plentiful, even outstripping the current demand.

"Where we are now is the best case scenario, in terms of what can happen when you actually commit the tools you have to fight an outbreak," says Dr. Boghuma Titanji, an infectious disease specialist at Emory University.

So what changed the trajectory?

Health experts attribute the success to changes in behavior among those at high risk for monkeypox and quick uptake of vaccines. But a growing body of evidence suggests another factor is also helping slow down the outbreak: the virus can spread only under very particular circumstances.

Monkeypox not likely to spread through saliva and surfaces

Initially, there was a lot of concern that monkeypox could spread widely at daycares or in schools, but, overall, there has been very little spread among children.

Only about 0.2% of U.S. cases have been in kids under 16. And, so far, there's no evidence that a sick child or teacher has spread the virus to another person at a school or daycare center. (College campuses are a separate issue. Several campuses have reported cases among students, but none have reported large outbreaks, at this point).

With children, the concern stemmed from the understanding that the virus can spread through saliva – meaning it can spread when you're up close in somebody's face while talking or coughing. Monkeypox can also spread when a person touches objects and surfaces that were contaminated by someone with an infection. But in reality, it's quite rare to catch monkeypox in either of these ways.

Several studies have found that often there isn't very much virus in the upper respiratory tract. Instead, the highest levels of virus occur on sores found on the skin and inside the anus.

In one of those studies, researchers at Israel Institute of Biological Research measured the levels of virus in 44 monkeypox patients. They took samples from lesions on the patients' skin and swabs of their throats. They found that the skin lesions contained 17-times the infectious virus particles, on average, than the swabs from the throat.

Another study, published last month, obtained similar results, but these researchers went even further: they also analyzed levels of viral DNA in patients' blood, urine, semen, and swabs from the anus, in addition to samples taken from the skin and throat. Although high levels of monkeypox DNA occurred in a few people's throats and semen, those samples, on average, contained much less virus than the samples from the anus or lesions on the skin. Urine and blood contained the lowest level of virus.

Together with a few previous studies, these newer findings explain why monkeypox is spreading almost exclusively through contact during sex, especially anal and oral sex, during the current outbreak.

Even with sex, monkeypox only causes outbreaks in particular circumstances

But this research still doesn't explain why the disease hasn't spread widely in women who, after all, are having sex, too.

Since the beginning of the U.S. outbreak, the CDC data show that the vast majority of cases have remained in men. The rates of infection are also very high among transgender men and women. Meanwhile, only about 2% of cases have occurred in women.

Turns out, it takes more than just having sex to keep a monkeypox outbreak going.

In a study, published last month in the journal Science, researchers found that monkeypox spreads at very different rates in different groups of people – and that rate depends greatly on people's sexual activity. Researchers from Nagasaki University and London School of Hygiene & Tropical Medicine built a mathematical model of the global outbreak. Then they looked to see how the number of sexual partners alters the transmission of monkeypox in their network. Overall, monkeypox outbreaks were highly likely in only one particular type of sexual network: where a small number of people have a high number of sexual partners.

Outside of that, outbreaks of monkeypox are very rare. The study found chains of transmission nearly always stop on their own because the chance of a person spreading the virus to another person is low.

In the U.S. outbreak, monkeypox "has really been contained in a core group of sexually active men who have sex with men, with multiple partners," says Dr. Jeffrey Klausner, a professor of medicine and public health at the University of Southern California who was not involved with the study. And within this group, some are at higher risk: "It's not in those long-term monogamous relationships, or men who have an additional occasional partner every couple of months ...This is really [concentrated among] men who have multiple new partners every week."

And as people within these active sexual networks acquire immunity to monkeypox – either by recovering from infections or getting vaccinated – the number of people who are suspectible to infection is falling.

Can monkeypox be eliminated in the U.S.?

Some say it may be possible to stop monkeypox transmission chains in the U.S. and bring cases down to nearly zero (at least, not counting the cases that will come from other parts of the world where the virus is still spreading).

"I think we can expect to see regional elimination, potentially national elimination, where we would not see a sustained number of cases," says Klausner, who points out that in some large cities, including San Francisco, Chicago and New York, only a few cases are being detected each day.

Still, new infections have not declined evenly across all cities, and the proportion of cases among men of color is rising.

In fact, CDC data show that monkeypox cases are down dramatically in white men, but nearly 70% of cases are now being detected in Black or Latino men. Health officials have acknowledged that these populations are getting vaccinated at lower rates than white men.

And not all experts are as optimistic about eliminating the virus domestically.

There are still key unknowns that could affect the trajectory of the outbreak, such as whether people without symptoms (or only mild symptoms) are spreading monkeypox unknowingly, and how well the vaccines work – both at preventing monkeypox infections and transmission. "Do they offer 100%, 75% or 50% protection?" Titanji says. "And is that protection for the rest of your life? Or will you need subsequent vaccination to maintain a level of protection?"

These are open questions.

Then there's the virus itself and how it could change. Could it gain a foothold among animal populations in the U.S. or Europe and establish a permanent virus reservoir? Or could it mutate in ways that may increase its infectiousness to humans?

"These are factors that will potentially impact whether we are able to eliminate this virus and go back to where we were before," Titanji says, "So the jury's still out."

Still, monkeypox may not need to be completely eliminated in the U.S. for it to no longer be a national public health emergency. As cases decline in major cities, outbreaks may be more limited and localized.

If that happens, the challenge will be making sure it doesn't "become another neglected, sexually transmissible infection, joining the ranks of gonorhea, chlamydia and syphilis," Titanji says. These are all preventable diseases, but they still cause long-term health problems for many people, particularly in historically marginalized communities.