New -- though limited -- data released on Wednesday morning from the Centers for Disease Control and Prevention found that people who haven't been vaccinated against monkeypox were 14 times more likely to get infected than vaccinated people.
The data is drawn from people who were eligible for a monkeypox vaccine in their states, which mostly includes men who have sex with men or people who have multiple sexual partners. The numbers provide the first sense of real-world effectiveness on the JYNNEOS vaccine, the leading inoculation against monkeypox.
"These new data provide us with a level of cautious optimism that the vaccine is working as intended," CDC Director Rochelle Walensky said in a White House briefing on monkeypox on Wednesday.
Dr. Demetre Daskalaskis, the monkeypox deputy response coordinator for the White House, urged people to share the "early good news."
"This knowledge is power -- and allows people to make more informed decisions about their health and build confidence in this important two dose vaccine," he said.
Real-world data on the JYNNEOS vaccine, initially designed to fight smallpox, has been limited since the start of the current outbreak
The Food and Drug Administration, the CDC and National Institutes of Health have -- largely at the behest of advocates in the LGBTQ community -- launched many studies on the JYNNEOS vaccine's safety and effectiveness since the outbreak began in May.
And though the new data is promising, Walensky also emphasized that it's just a glimpse. There are still outstanding questions about the full breadth of JYNNEOS' protection.
For example, the new efficacy rate of the vaccine was based on data gathered two weeks after the first vaccine, not after a full course of two shots, 28 days apart.
The CDC has yet to release data on efficacy after full vaccination, even though two shots is the consensus recommendation among federal public health officials.
"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. That said, we know from laboratory studies that immune protection is highest two weeks after the second dose of vaccine," Walensky said on Wednesday.
"And it is for that reason that we continue, even in light of these promising data, to strongly recommend people receive two doses of JYNNEOS vaccine spaced out 28 days apart to ensure durable, lasting immune protection against monkeypox," she said.
More studies on efficacy after two shots are underway, Walensky said.
The CDC also has yet to differentiate the efficacy data between injection methods, which would provide further clarity on any potential differences between the new approach, where a smaller dose is injected just under the skin, and the original approach, where the injection was deeper.
It's also not clear how much of a role behavioral changes might be playing in the data on the vaccine's effectiveness.
If vaccinated people are reducing their sexual partners or chances for skin-to-skin contact, that could decrease their monkeypox infection rates, too.
Walensky pledged that more data is coming but that CDC is working to provide all the data it has in real time as soon as it is available -- a lesson learned from COVID-19.
"[The] CDC will continue to evaluate how these vaccines are working in the current outbreak through a portfolio of vaccine effectiveness projects that will help us understand the level of protection provided and how long that protection lasts. And we will continue to deliver these data to you as soon as we have them," Walensky said.
The CDC is also expanding eligibility for monkeypox vaccines to include more at-risk Americans -- though the JYNNEOS vaccine is still not recommended for the general population.
The expansion now includes gay or bisexual men who have had one partner in the last six months or received a new diagnosis of one or more sexually transmitted diseases in the last six months. It also includes sex workers.
Though many states and jurisdictions have already expanded eligibility, the CDC's latest move puts it in closer alignment with local guidance.