Now the herd is restless. And experts at the Centers for Disease Control and Prevention have set aside herd immunity as a national goal.

The prospects for meeting a clear herd-immunity target are "very complicated," said Dr. Jefferson Jones, a medical officer on the CDC’s COVID-19 Epidemiology Task Force.

“Thinking that we’ll be able to achieve some kind of threshold where there’ll be no more transmission of infections may not be possible,” Jones acknowledged last week to members of a panel that advises the CDC on vaccines.

Vaccines have been quite effective at preventing cases of COVID-19 that lead to severe illness and death, but none has proved reliable at blocking transmission of the virus, Jones noted. Recent evidence has also made clear that the immunity provided by vaccines can wane in a matter of months.

The result is that even if vaccination were universal, the coronavirus would probably continue to spread.

“We would discourage” thinking in terms of “a strict goal,” he said.

To Dr. Oliver Brooks, a member of the CDC's Advisory Committee on Immunization Practices, it was a sobering new message, with potentially worrisome effects.

With just 58.5% of all Americans fully vaccinated, “we do need to increase" the uptake of COVID-19 shots, said Brooks, chief medical officer of Watts Healthcare in Los Angeles. Unfortunately, he said, Jones' unexpected admission "almost makes you less motivated to get more people vaccinated.”