By Lisa Jarvis / Bloomberg Opinion

As U.S. public health authorities deploy a new strategy to expand access to the monkeypox vaccine, they need to be more clear and open about exactly what protection the shots can provide.

The Food and Drug Administration this week issued emergency authorization to administer a small dose of Bavarian Nordic’s Jynneos vaccine — one-fifth the usual size — within the skin, or intradermally, rather than under the skin. About 440,000 of the 600,000 recently allocated doses of Jynneos remain, an official from the Department of Health and Human Services explained, and the new strategy will stretch that supply into 2.2 million shots.

In making this change, the FDA is relying on a 2015 study showing that the two methods of administration elicit a similar immune response in healthy adults. Unfortunately, that study provides no information about effectiveness of either approach.

Neither does the statistic so often cited, including by the World Health Organization, that the vaccine is effective at preventing 85 percent of monkeypox infections. That figure is based on data collected in Africa in the 1980s, when an older monkeypox vaccine (one that elicits a similar immune response) was used and the primary form of transmission was either animal or household exposure.

These immunological studies give public health experts confidence that the vaccine protects against the most severe cases of monkeypox, but no clinical data exist to show whether it stops transmission, prevents the painful lesions seen in most cases, or provides lifelong protection of any kind.

Until such data are gathered, public health officials need to be clear about what they don’t know; especially when speaking to people considered at highest risk in the current outbreak, gay and bisexual men.

Right now, the biggest concern is communicating the risk of infection after vaccination. “How do we accurately message to right now mostly men what their risk is after being fully vaccinated?” said Jay Varma, a professor of population health sciences at Weill Cornell Medicine, in New York City.

We’ve seen this movie before. In the summer of 2021, Moderna’s and Pfizer’s covid vaccines appeared to prevent not just the most serious cases of disease but also infection. Vaccinated people were told they could feel safe gathering indoors, without masks; that the country was heading toward “normal.” Then came the wave of breakthrough infections.

Infectious-disease experts, including Varma, who served as New York City Mayor Bill de Blasio’s senior adviser for public health, “got a little bit ahead of our skis,” he said. That bred confusion about what activities were actually safe, and ultimately undermined confidence in the covid vaccines.

Communicating the value of vaccines is a balancing act. People need accurate information about how safe shots are and exactly what protection they provide. Unknowns and nuances need to be shared so people can make their own best lifestyle decisions after vaccination. The goal should be to encourage vaccination but not exaggerate its benefits.

Messaging about Jynneos has been further muddied by efforts in some cities to stretch limited vaccine supplies by offering people one shot instead of two. While that gets more shots in arms faster, the danger is that men might feel safe to resume sexual activity without actually having sufficient protection from the monkeypox virus.

Rochelle Walensky, director of the Centers for Disease Control and Prevention, this week reiterated the recommendation that people take steps to protect themselves from exposure to the virus, especially if they have had only one shot, “because we don’t yet know how well these vaccines work.”

The CDC has been working on several vaccine effectiveness projects that will gather data from both on-the-ground investigations and formal clinical trials, she said. The agency expects to have initial estimates of vaccine effectiveness soon. The National Institutes of Health is also mapping out its own studies.

During the pandemic, people in the LGBTQ community — and men in particular — had higher levels of covid vaccination than heterosexual adults. The CDC and FDA should now do everything possible to support that commitment to personal and public health by providing clear communication about the monkeypox vaccine.

Lisa Jarvis is a Bloomberg Opinion columnist covering biotech, health care and the pharmaceutical industry. Previously, she was executive editor of Chemical & Engineering News.