Many Americans now can get a second COVID-19 booster, but it's hard to tell who really needs another shot right now and who could wait.

The Food and Drug Administration authorized extra Pfizer or Moderna shots for anyone 50 or older and for some younger people with severely weakened immune systems. It's an effort to get ahead of a possible next coronavirus surge.

With COVID-19 cases low in the U.S., it's easy to ignore calls for another dose — or for those who aren't yet vaccinated or boosted to get up to date, said Dr. Erica Johnson, an infectious disease specialist at the American Board of I...