Why San Francisco Is ‘Cautiously Optimistic’ The Monkeypox Outbreak Is Slowing

After about two months of rapid spread, San Francisco appears to be turning a corner on monkeypox, with early data showing the local epidemic may be slowing down.

The number of new cases reported each week hit a high of 143 the week of July 24 and has tapered each week since, first to 87 cases, then 54 and then, last week, to fewer than five, according to figures provided by the San Francisco Department of Public Health.

Because data for the past two weeks is preliminary, San Francisco health officials are reluctant to say for certain that the outbreak is receding. Still, a downward trajectory would be consistent with a slowdown of new cases reported in the United Kingdom, parts of Canada and New York City. Cases across California are also edging down, according to the Chronicle monkeypox tracker, which uses data from the California Department of Public Health.

“It is hopeful the way the numbers are shaping up,” said San Francisco Health Officer Dr. Susan Philip. “It’s certainly better than seeing case numbers double every week.”

Though officials and experts are cautious about calling the arc of the epidemic, they note that many people in high-risk groups have now been vaccinated and that some health care providers are reporting fewer patients needing hospital-level care.

Anecdotally, some of the people most likely to contract the virus seem to be modifying their behavior to avoid catching and spreading the disease, which is rarely fatal but can cause painful lesions.

Philip underscored that she’d like to see a consistent downward trend for at least three to four consecutive weeks — so far, it’s only been one week with complete data, and two additional weeks of preliminary data — before she’d feel comfortable calling it a definitive decline.

“Sometimes we believe a curve is going down and see it going back up again,” she said. “We see that with COVID. That’s another reason we’re cautiously optimistic but we need to watch closely before we make clear determinations.”

The concentration of monkeypox DNA detected at sewer sheds in and around San Francisco has been holding steady the past few weeks, which would be consistent with a leveling off of new cases. In contrast, earlier in the epidemic, there was a clear increase in concentration in the Southeast San Francisco treatment plant, in particular, which began to plateau in early August, said Alexandria Boehm, a Stanford civil and environmental engineering professor who co-leads the Sewer Coronavirus Alert Network (SCAN). The network began as a way to detect the presence of SARS-CoV-2 in wastewater and now also tracks influenza A, RSV and monkeypox.

While it’s uncertain the slowdown in new cases will continue, local infectious disease experts say it was expected that monkeypox infections would eventually taper and start to come down.

The epidemic has mostly spread through sexual networks of men who have sex with men — especially those with multiple sexual partners — so the community at highest risk is fairly well-defined, and there hasn’t been significant spillover to the general population outside those networks. Once those highest-risk individuals have either been infected or vaccinated, the virus runs out of people to target. Scientists believe that once someone is infected with monkeypox, they likely have long-lasting if not lifelong immunity.

“Once it finds all the people it’s going to infect, there’s no more people to infect,” said UCSF infectious disease expert Dr. Peter Chin-Hong. “It’s not gone away, but it’s finding fewer fresh blood, so to speak.”

There is also evidence that the highest-risk people have implemented behavioral changes, including reducing the number of their sexual partners, in light of the outbreak, said Dr. Abraar Karan, an infectious disease physician at Stanford who works at an HIV clinic.

“I talk to patients who identify as men who have sex with men, and they’ve indicated they’re well aware of the outbreak, are interested in getting vaccinated, and have reduced sexual contacts and exposures,” Karan said. “For populations who have dealt with HIV, they’re very well-versed in how devastating an outbreak can be and understanding the importance of risk-reducing activities.”

Chin-Hong, who’s been treating monkeypox patients, said anecdotally that the number of patients seeking care for the most severe cases seems to be decreasing.

“Two weeks ago we probably had the most people seeking treatment,” he said. “Last week and this week, there are still a fair number but not many. This week is probably the least I’ve seen for awhile.”

Chin-Hong said the apparent decline may be linked to the timing of large gay Pride events. The bulk of transmission may have occurred in late June when New York City and San Francisco held their annual Pride parties.

Even if the overall downward trend holds true, the virus will likely continue to circulate — especially among higher-risk populations like sex workers, homeless people, and groups in jails, prisons and potentially college campuses where sexual networks tend to be more active than in the general population, Karan said.

“I’m not surprised caseloads have come down, I expect that would happen as we focus resources on high-risk communities,” he said. “But I don’t think this is the end of the outbreak.”


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