With Pride coming up, the San Francisco Department of Public Health and the federal Centers for Disease Control and Prevention are urging people at risk for mpox, including sexually active gay and bisexual men and people living with HIV, to receive two doses of the Jynneos vaccine to protect themselves from the disease. It comes as concern grows about a deadlier version of mpox has been found in Africa.
"With summer celebrations such as Pride approaching, now is a great time to protect yourself against mpox by getting vaccinated. The mpox vaccine is available through health systems and at clinics," San Francisco Health Officer Dr. Susan Philip stated in a DPH news release. "Even if you are fully vaccinated, it is still important to remain diligent since no vaccine is 100% effective. If you are experiencing symptoms of mpox such as a rash that looks like pimples or blisters, talk to your health care provider about getting tested, and talk to your partners so they can be informed and prevent the spread of infection."
Mpox cases have risen in the United States this year compared with 2023, though they remain far below the level seen at the peak of the outbreak in the summer of 2022. Cases in San Francisco remain low and stable. But a growing outbreak of a more deadly mpox strain in the Democratic Republic of the Congo raises concerns about wider international spread.
"SF DPH is closely monitoring global mpox activity, including the clade I mpox outbreak in the Democratic Republic of Congo," SF DPH told the Bay Area Reporter in a statement. "Being fully vaccinated remains the best way for people to protect themselves against mpox infection, and we are encouraging people to seek the mpox vaccine ahead of the summer season."
Mpox is transmitted mainly through close skin-to-skin contact, including sex. The global outbreak that started in May 2022 primarily affected gay and bisexual men and others in their sexual networks. As of early March, the CDC had identified more than 32,000 cases in the U.S., resulting in 58 deaths. This outbreak involved clade II mpox, which causes less severe illness than clade I. The U.S. case fatality rate is low (around 0.3%), but mpox is deadlier for people with advanced HIV.
The global outbreak declined dramatically thanks to a combination of natural immunity after infection, vaccination, and behavior change, but mpox continues to circulate at a low level. According to the May 23 Morbidity and Mortality Weekly Report, a total of 1,802 confirmed or probable cases were reported to the CDC between October 2023 and the end of April 2024. More than 90% were among men who have sex with men and around half were living with HIV. There were about 60 cases reported per week during this period, down from a high of about 3,000 in July 2022.
While this is good news, cases have increased in 2024 compared with 2023. Nearly 750 cases have been reported so far this year, more than double the number at the same time last year. Nearly all regions have reported more cases this year, albeit with substantial local variation. New York City, for example, has seen 191 cases in 2024, while San Francisco has identified only nine cases — and none since late April.
Mpox in the DRC
Prior to the 2022 global outbreak, mpox was known as an uncommon disease in western and central Africa. It was typically linked to contact with wild animals, and it was not thought to spread easily between people.
Clade I mpox has long been endemic in the DRC, but cases began to increase last year. Between January 2023 and April 2024, there were 19,919 suspected cases and 975 deaths, for a case fatality rate of 4.9%, according to the May 16 Morbidity and Mortality Weekly Report. However, due to widespread poverty, lack of health infrastructure and ongoing armed conflict, most suspected cases have not been laboratory confirmed.
The DRC appears to be experiencing concurrent mpox outbreaks. In some parts of the country, around two-thirds of suspected cases and most deaths have been among children. In keeping with historical patterns, this is likely due to multiple separate exposures to infected animals and subsequent chains of household transmission.
At the same time, some cities and towns are seeing cases primarily among adults, apparently driven by sexual transmission. A man suspected of being the first in a cluster of cases near the capital Kinshasa visited underground clubs frequented by gay men, and he reported several sexual contacts with both men and women. Across the country, an outbreak in Kamituga, a mining town near the border with Rwanda and Burundi, appears to be mainly driven by heterosexual contact. Many of the cases involve sex workers. Genomic analysis revealed a distinct lineage of clade Ib mpox that appears to spread more easily from person to person.
"These new features of sexual transmission now raise additional concerns over the continuing rapid expansion of the outbreak in the country in a nationally and internationally mobile key population," according to the World Health Organization. "The risk of mpox further spreading to neighboring countries and worldwide appears to be significant." The similarly named Republic of Congo, which borders the DRC to the west, is already starting to see an uptick in cases.
Infectious diseases don't remain confined to one country, and the DRC outbreak underscores the need to make mpox vaccines and treatment available worldwide, officials noted. While the Jynneos vaccine has been widely deployed in the U.S. and other high-income countries, it is still not readily available in Africa.
Public health response
No cases of clade I mpox have been detected so far in the U.S. or in any other countries outside of endemic areas in Africa. The CDC and other laboratories have tested around 1,200 mpox specimens and wastewater samples from 186 sites, all of which were negative for this strain. The CDC urges clinicians and health departments to be alert for clade I mpox among travelers to the DRC. If a case is suspected, they should request clade-specific testing. Standard tests can detect both mpox clades but usually can't tell them apart.
In a May 10 rapid risk assessment, the CDC deemed the risk posed by the DRC outbreak to be "very low" for the general population in the U.S. and "low to moderate" for gay and bisexual men and those in their sexual networks. Sustained heterosexual transmission of clade I mpox and widespread transmission among children are considered unlikely due to the absence of animal reservoirs, smaller household sizes, and better access to health care and sanitation resources.
The CDC now recommends routine vaccination for people at risk for mpox, whether or not an outbreak is underway. According to SF DPH criteria, this includes "all people living with HIV, anyone taking or eligible to take HIV PrEP, and all men, trans people, and nonbinary people who have sex with men, trans people, or nonbinary people," as well as anyone else who wants protection from mpox.
To date, only 25% of eligible individuals have received two doses of the Jynneos vaccine, and most recent mpox cases have involved people who were not fully vaccinated. Of the 32,819 confirmed or probable cases reported between May 2022 and May 2024, three-quarters were unvaccinated. Among those who received two doses, however, the risk of mpox infection is estimated at around 0.1%. Among those with breakthrough infections, fully vaccinated people had milder illness, were less likely to be hospitalized, and none died.
When Jynneos was in short supply, the U.S. Food and Drug Administration authorized an intradermal injection method that allows a single vial to be split into five doses. Recent studies have shown that this dose-sparing method and standard subcutaneous injections generate equivalent antibody responses. Researchers also saw no difference in immune responses between HIV-negative people and HIV-positive people with a CD4 count above 200. Antibody levels do decline within months after the second dose, but this is not the only marker of protection. The real-world CDC findings "suggest that immunity is not waning."
According to CDC guidelines, either the subcutaneous or intradermal vaccination method may be used, but the former is preferred if supply is adequate. The recommended interval between shots is four weeks, but people who previously received one dose — no matter how long ago — do not need to restart the series. At this time, additional booster doses are not recommended. People who have already had mpox do not need to be vaccinated. Studies indicate that Jynneos and the antiviral drug TPOXX (tecovirimat) are effective against both clade I and clade II mpox.
Jynneos is now commercially available, but some public health sites in San Francisco still offer it for free.
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