Police, the group wrote, needed the vaccine "to keep them, and the public with whom they interact, safe from infection."

But to the group's surprise, officers did not rush to get the shot. And months later, with the vaccine widely available across the country, scores remain unvaccinated.

"We worked very hard, along with others, to ensure that police officers had early availability on a premise that they'd all want it," said James Pasco, FOP executive director.

Nearly a quarter of Americans age 18 and older remain unvaccinated, according to a Washington Post analysis of federal data, frustrating officials and fueling bitter debates. Yet the continued resistance among the first responders included in those tens of millions is particularly troubling and creates a different kind of threat, experts say.