The Centers for Disease Control and Prevention on Thursday urged gay men and other individuals at high risk from mpox to get fully vaccinated to prevent a summer resurgence of the virus.
The CDC's call for those at risk to get up to date on their vaccines comes after a cluster of at least 21 mpox cases was reported in the Chicago area this month.
Many of the people who caught mpox in the Chicago cluster were partially or fully vaccinated against the virus, raising questions about whether immunity from the shots might wane over time.
The patients in the Chicago cluster all have mild symptoms, said Demetre Daskalakis, deputy head of the White House mpox task force, on a call with reporters Thursday.
Daskalakis said no vaccine is perfect but people who have received two doses have a much lower risk of catching and spreading the disease. Vaccination also lowers the risk of severe illness, hospitalization and death, even if the shots do not always prevention infection, he said.
More than 30,000 mpox cases and 42 deaths have been reported in the U.S. since May 2022, according to the CDC. The outbreak has primarily affected gay, bisexual and other men who have sex with men.
Daskalakis said most new cases of mpox continue to be reported among men who have sex with men.
New cases have declined dramatically since the peak last August, but the Chicago cluster has raised concerns that the virus could come back this summer.
Less than a quarter of the 1.7 million people at highest risk from mpox have received two doses of the vaccine. These individuals are primarily gay and bisexual men living with HIV or who are taking drugs to prevent HIV infection, called pre-exposure prophylaxis.
"The chances of renewed outbreaks go up when fewer people have been vaccinated," said Dr. Christopher Braden, the CDC's mpox incident manager.
Vaccine effectiveness
Three new reports released Thursday by the CDC, the New York State Department of Health, and Epic Research in the New England Journal of Medicine demonstrated that two shots of the Jynneos vaccine provide more protection than a single dose.
The CDC estimated in a study that a single dose of the vaccine is 75% effective at preventing mpox, while two doses were about 86% effective. The New York health officials found similar results in a second study, with one dose 68% effective and two doses about 88% effective.
But the New England Journal of Medicine study found that one dose was just 36% effective at preventing mpox, while two doses were 66% effective.
Though the estimates of mpox vaccine effectiveness vary, Daskalakis said the message is clear: "One dose is good, two doses are better."
"Now is the time to get vaccinated," Daskalakis told reporters on a call Thursday. "If you didn't get your first dose — get it. If you didn't get your second dose – get that," he said.
People with weak immune systems who received two doses had 70% protection against mpox, while people with healthy immune systems had about 88% protection, according to the CDC study.
The effectiveness of the vaccine was largely the same regardless of whether the shots were administered through subcutaneous or intradermal injection or a mix of the two, the CDC found.
A subcutaneous injection is administered under the skin, while an intradermal shot is injected between the layers of the skin. The intradermal shots use less vaccine substance, which allows public health authorities to stretch the vaccine supply.
Does immunity wane?
Braden said the CDC is conducting studies to determine whether immunity after vaccination might wane over time, and scientists hope to learn more from this data soon.
Braden said waning immunity is only 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 cluster received were compromised in some way or weren't administered properly, he said.
The CDC recommends mpox vaccination for gay and bisexual men, as well as transgender and nonbinary people who have had more than one sex partner or a new STD diagnosis in the past six months.
Vaccination is also recommended for anyone who has had a known or suspected exposure to mpox. People who have had sex for money, and those who have had sex at a commercial sex venue or at a large public event in an area where mpox is spreading, should also get vaccinated.