Americans who received just one dose of the monkeypox vaccine were much less likely to be infected by the virus, according to preliminary data published by the Centers for Disease Control and Prevention on Wednesday.
The data — compiled from 32 states from July to September — is the first analysis released by public health officials into the vaccine’s effectiveness when monkeypox was largely spreading across LGBTQ+ communities this summer.
Those who didn’t receive a single dose of the vaccine were 14 times more likely to be infected than those who had, officials said.
They stressed that immune protection was best at two weeks after the second dose of a monkeypox vaccine. Current guidelines suggest those at risk receive two shots 28 days apart to provide “durable, lasting immune protection.”
“These early findings and similar results from studies in other countries suggest that even one dose of the monkeypox vaccine offers at least some initial protection against infection,” CDC Director Rochelle Walensky said during a press briefing Wednesday. “That said, we know from laboratory studies that immune protection is highest two weeks after the second dose of vaccine.”
“These new data provide us with a level of cautious optimism that the vaccine is working as intended,” she added.
About 800,000 doses have been administered across the country after a frantic period when at-risk communities were scrambling to get the shots. That figure is far short of the estimated 1.6 million Americans thought to be at high risk of monkeypox. Other reports show that Black and Hispanic men have received a disproportionately low number of the shots, too.
Cases have declined in recent weeks, but Walensky said the recent data was encouraging and the CDC would expand eligibility to include preventative vaccination for vulnerable communities.
The U.S. has seen more than 25,000 cases of monkeypox since the outbreak began in May, including one death. Most of those cases have been among gay and bisexual men, and officials hope the new pre-exposure prophylaxis (PrEP) strategy could help reach hundred of thousands of people who have yet to get the vaccines.
“By expanding eligibility and shifting to a PrEP strategy across the country, we are looking to ensure those who are at the highest risk for monkeypox receive the vaccine before exposure and that vaccines continue to be made available equitably to those who need them,” she said.
The findings released this week still have some gaps: The data doesn’t include details about the severity of cases in vaccinated people who contracted monkeypox or how long the vaccine may provide protection. There is also no data showing how behavioral changes may have prevented infection.
“We’re going to need a little bit more time and a bit more numbers,” Walensky said Wednesday. “This is really in an effort to give you all the data as soon as we have it.”