Since the earliest days of the pandemic, there has been one collective goal for bringing it to an end: achieving herd immunity. That's when so many people are immune to a virus that it runs out of potential hosts to infect, causing an outbreak to sputter out.

Many Americans embraced the novel farmyard phrase, and with it, the projection that once 70% to 80% or 85% of the population was vaccinated against COVID-19, the virus would go away and the pandemic would be over.

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. ...