Smallpox and monkeypox MPXV booster vials.
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this CDC Gay men and other high-risk groups were urged on Thursday to get the mpox vaccine to prevent a summer resurgence of the virus.
After clusters of at least 21 mpox cases were reported in the Chicago area this month, the CDC is urging high-risk groups to get vaccinated.
Many people who contracted mpox in the Chicago cluster were partially or fully vaccinated against the virus, raising questions about whether vaccine immunity wanes over time.
The patients in the Chicago cluster all had mild symptoms, Demetre Daskalakis, deputy head of the White House mpox task force, said in a call with reporters Thursday.
No vaccine is perfect, but people who have had two doses of the vaccine have a much lower risk of catching and spreading the disease, Daskalakis said. Vaccinations also reduce the risk of serious illness, hospitalization and death, even though shots don’t always prevent infection, he said.
Since May 2022, more than 30,000 cases of mpox and 42 deaths have been reported in the United States, according to the CDC. The outbreak primarily affects gay, bisexual and other men who have sex with men.
Most new mpox cases continue to be reported among men who have sex with men, Daskalakis said.
New cases have fallen sharply since their peak last August, but the Chicago cluster has raised concerns that the virus could make a comeback this summer.
Of the 1.7 million people most at risk for mpox, less than a quarter had received two doses of the vaccine. These are mainly gay and bisexual men who are HIV-infected or who are taking medicine to prevent HIV infection, called pre-exposure prophylaxis.
“When fewer people are vaccinated, the likelihood of another outbreak increases,” said Dr. Christopher Braden, the CDC’s mpox incident manager.
Vaccine Effectiveness
Three new reports published Thursday by the CDC, the New York State Department of Health and Epic Research in the New England Journal of Medicine suggest that two shots of the Jynneos vaccine provide more protection than one shot.
The CDC estimated in one study that a single dose of the vaccine was 75% effective in preventing mpox, while two doses were about 86% effective. New York health officials found similar results in a second study, 68 percent effective with one dose and around 88 percent effective with two doses.
But the New England Journal of Medicine study found that one dose of preventive mpox was only 36 percent effective, compared with 66 percent for two doses.
While estimates of the effectiveness of the mpox vaccine vary, Daskalakis said the message is clear: “One dose is good, two doses is better.”
“Now is the time to get vaccinated,” Daskalakis told reporters by phone Thursday. “If you didn’t get your first dose — get it. If you didn’t get your second dose — get that,” he said.
According to the CDC, people with weakened immune systems were 70 percent protected from mpox after two doses, compared with about 88 percent in people with healthy immune systems.
The CDC found that the vaccine was about the same effective whether it was given subcutaneously or intradermally, or a combination of the two.
Subcutaneous injections are given under the skin, while intradermal injections are given between the layers of the skin. Intradermal injections use less vaccine material, which allows public health authorities to expand the vaccine supply.
Will immunity decline?
The CDC is conducting studies to determine whether immunity wanes over time after vaccination, and scientists hope to learn more from the data soon, Braden said.
Declined immunity is just one possible explanation for the number of people in the Chicago cluster who were accidentally infected with mpox even though they were fully vaccinated against the virus, Braden said.
The CDC is also looking at whether the virus might have evolved over time to overcome immunity, he said. It’s also possible that patients in the Chicago cluster had been given vaccines that were somehow compromised or not administered properly, he said.
The CDC recommends the mpox vaccine for gay and bisexual men, as well as transgender and nonbinary people who have had more than one sexual partner in the past six months or who have a newly diagnosed STD.
Vaccination is also recommended for anyone with known or suspected exposure to mpox. People who have sex for money and at commercial sex venues or at large public events in areas where mpox has been transmitted should also be vaccinated.