Bottle of booster vaccine against smallpox and monkeypox MPXV.
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The Centers for Disease Control and Prevention on Thursday urged gay men and others at high risk for Mpox to get fully vaccinated to prevent a summer resurgence of the virus.
The CDC’s call for at-risk individuals to keep up to date with their vaccinations comes after a cluster of at least 21 MPox cases was reported in the Chicago area this month.
Many of the people who contracted MPOX in the Chicago cluster were partially or fully vaccinated against the virus, raising the question of whether immunity to the vaccinations might wane over time.
The patients in the Chicago cluster all had mild symptoms, Demetre Daskalakis, deputy chief of the White House MPOX task force, said in a phone call with reporters on Thursday.
Daskalakis said no vaccine is perfect, but people who received two doses were at a much lower risk of catching and spreading the disease. Vaccination also reduces the risk of serious illness, hospitalization and death, although immunizations don’t always prevent infection, he said.
More than 30,000 MPOX cases and 42 deaths have been reported in the United States since May 2022, according to the CDC. The outbreak has mostly affected gay, bisexual, and other men who have sex with men.
Daskalakis said most new cases of Mpox continue to be reported in men who have sex with men.
New cases have fallen dramatically since peaking last August, but the Chicago cluster has raised concerns the virus could reemerge this summer.
Less than a quarter of the 1.7 million people most at risk of Mpox have received two doses of the vaccine. These people are primarily gay and bisexual men who are living with HIV or who are taking medication to prevent HIV infection, known as pre-exposure prophylaxis.
“The likelihood of new outbreaks increases when fewer people have been vaccinated,” said Dr. Christopher Braden, CDC MPOX incident manager.
effectiveness of the vaccine
Three new reports published Thursday in the New England Journal of Medicine by the CDC, the New York State Department of Health and Epic Research showed that two shots of the Jynneos vaccine offer more protection than a single dose.
The CDC estimated in a study that a single dose of the vaccine was 75% effective in preventing Mpoxen, while two doses were about 86% effective. The New York health authorities found similar results in a second study: one dose was 68% effective and two doses were about 88% effective.
However, the New England Journal of Medicine study found that one dose was only 36% effective in preventing smallpox, while two doses were 66% effective.
Though estimates of the effectiveness of the MPOX vaccine vary, Daskalakis said the message is clear: “One dose is good, two doses are better.”
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“Now is the time to get vaccinated,” Daskalakis said in a phone call to reporters on Thursday. “If you haven’t gotten your first dose – go get it. If you haven’t gotten your second dose – get that one,” he said.
People with weak immune systems who received two doses had a 70% protection against Mpox, according to the CDC study, while those with healthy immune systems had about 88% protection.
Vaccine efficacy was largely the same whether the shots were given by subcutaneous or intradermal injection, or a mixture of the two, the CDC found.
A subcutaneous injection is given under the skin, while an intradermal injection is given between the layers of the skin. Intradermal vaccines use less vaccine substance, allowing health authorities to expand vaccine supplies.
Is immunity fading?
Braden said the CDC is conducting studies to determine whether post-vaccination immunity might decline over time, and scientists hope to learn more from that data soon.
Braden said falling immunity is just one possible explanation for why an unexpectedly large number of people in the Chicago cluster contracted MPOX despite being fully vaccinated against the virus.
He said the CDC is also investigating whether the virus evolved over time to overcome immunity. It’s also possible that the vaccine the patients in the Chicago cluster received was compromised in some way or wasn’t administered properly, he said.
The CDC recommends the MPOX vaccination for gay and bisexual men, as well as transgender and non-binary people who have had more than one sexual partner in the past six months or who have had a new STD diagnosis.
Vaccination is also recommended for anyone who is known or suspected to have been exposed to Mpoxen. 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.
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