Add to that: There’s little past experience with the vaccine, known as Jynneos in the US, being used against this disease. It was only approved by the US Food and Drug Administration in 2019, primarily for the prevention of smallpox in case that virus—eradicated from circulation by an earlier vaccine, but retained in two labs—was ever used as a biological weapon. Jynneos underwent human safety studies but was never tested for efficacy against monkeypox in people; those estimates are based on animal work. It has never been available commercially in the US, but was instead held in the National Strategic Stockpile as a safer alternative to that older smallpox vaccine, which can cause dangerous reactions in people with damaged immune systems. It was released to health departments only on the rare occasions when an infected traveler accidentally carried the virus into the US.

As a result, “we have no estimate of vaccine efficacy, given the modes of transmission that we are dealing with, which are very different than the modes of transmission that we traditionally see,” Rimoin says.

Now, of course, Jynneos is being administered everywhere, but it’s too early to draw conclusions about how much immunity those shots are creating. The vaccination campaign has been uneven: At first, there was so much demand in big coastal cities that men lined up for hours, online appointment dashboards filled up in minutes, and to stretch supplies, clinics held back on the second doses that lock in immunity. In response, the White House proposed a dose-splitting strategy that increased availability, though at the cost of requiring a different injection technique that some health care workers were unfamiliar with. Now, clinics in cities that were swamped by the first wave, such as New York, are posting thousands of new appointments regularly and are able to give second doses.

In some areas, in fact, there may be an oversupply. “Our demand has gone down significantly,” says Philip Huang, a physician and director of the Dallas County, Texas Health and Human Services Department. “We have empty appointments every day. We’re giving second doses.”

Health departments based their appointment offerings and vaccine allocation requests on their sense of how many men who have sex with men live in their communities, so those open appointments suggest that everyone who is eligible for protection may not be receiving it. They may not know they are at risk, they may fear the stigma of stepping forward, or they may not be aware the vaccine is available because overworked health departments do not have the time or personnel to craft precise messages to hard-to-reach groups. “We are still in an active public health emergency, a pandemic response, and our health department staffs are tired,” says Lori Tremmel Freeman, CEO of the National Association of County and City Health Officials.

And in a White House briefing this morning, federal health officials acknowledged those departments are strapped for cash to create their campaigns, in some cases moving money from HIV and STD programs to pay for vaccination workers and push out education. “Our local jurisdictions have received no resources specific for monkeypox,” CDC director Rochelle Walensky said. “It speaks to the need for supplemental funds.”