Vial of booster vaccine against smallpox and monkeypox MPXV.
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On Thursday, the Centers for Disease Control and Prevention urged gay men and others at high risk for mpox to get fully vaccinated to avoid a summer resurgence of the virus.
The CDC’s call for people at risk to get up to date on their vaccines comes after a cluster of at least 21 cases of mpox was reported in the Chicago area this month.
Many people who caught mpox in the Chicago cluster were fully vaccinated against the virus, raising questions about whether immunity to injections might wane over time.
The patients in the Chicago group all have mild symptoms, according to Demetre Daskalakis, deputy head of the White House mpox task force.
Daskalakis said no vaccine is perfect, but people who have received two doses have a much lower risk of catching and spreading the disease.
Three new reports published Thursday by the CDC and the New England Journal of Medicine have shown that two injections of the Jynneos vaccine provide more protection than a single dose.
The CDC estimated in one study that a single dose of the vaccine is 75% effective in preventing mpox, while two doses were about 86% effective. The New York State Department of Health found similar results in a second study, with one dose 68% effective and two doses approximately 88% effective.
But the New England Journal of Medicine study found that one dose was only 36% effective in preventing mpox, while two doses were 66% effective.
Although 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 on a call Thursday. “If you didn’t get your first dose, take it. If you didn’t get your second dose, take it,” he said.
The effectiveness of the vaccine was largely the same whether the injections were given by subcutaneous or intradermal injection or a mixture of the two, according to the CDC.
A subcutaneous injection is given under the skin, while an intradermal injection is injected between the layers of the skin. Intradermal injections use less vaccine material, allowing public health authorities to stretch the vaccine supply.
Less than a quarter of the 1.7 million people most at risk of mpox – mostly gay, bisexual and other men who have sex with men – received two doses of the vaccine.
Daskalakis said most new cases of mpox continue to be reported among men who have sex with men.
CDC mpox incident manager Dr. Christopher Braden said health authorities are conducting studies to determine if immunity after vaccination might wane over time.
Braden said the waning immunity is just one possible explanation for why an unexpected number of people in the Chicago cluster caught mpox despite being fully vaccinated against the virus.
He said the CDC is also studying whether the virus may have evolved over time to overcome immunity. It’s also possible that the vaccine the patients in the Chicago group received was compromised in some way or wasn’t administered correctly, he said.
The CDC recommends mpox vaccination for gay and bisexual men as well as transgender and non-binary people who have had more than one sexual partner or been newly diagnosed with an STD in the past six months.
Vaccination is also recommended for anyone with known or suspected exposure to mpox. People who have had sex for money and those who have had sex in a commercial sex venue or at a large public event in an area where mpox is spreading should also get vaccinated.
This is a developing story. Please check for updates.