Americans 50 and older can get a second COVID-19 booster if it’s been at least four months since their last vaccination, a chance at extra protection for the most vulnerable in case the coronavirus rebounds.

The Food and Drug Administration on Tuesday authorized an extra dose of the Pfizer or Moderna vaccine for that age group and for certain younger people with severely weakened immune systems.

The Centers for Disease Control and Prevention later recommended the extra shot as an option but stopped short of urging that those eligible rush out and get it right away.

Dr. Rochelle Walensky, CDC’s director, said it was especially important for older Americans - those 65 and older - and the 50-somethings with chronic illnesses such as heart disease or diabetes to consider another shot.

“They are the most likely to benefit from receiving an additional booster dose at this time,” Walensky said.

There’s evidence protection can wane particularly in higher-risk groups, and for them another booster “will help save lives,” FDA vaccine chief Dr. Peter Marks said.

For all the attention on who should get a fourth dose of the Pfizer and Moderna vaccines, only about half of Americans eligible for a third shot have gotten one - and the government urged them to get up to date. Two shots plus a booster still offer strong protection against severe illness and death, even during the winter surge of the super-contagious omicron variant.

The move toward additional boosters comes at a time of great uncertainty, with limited evidence to tell how much benefit an extra dose right now could offer. COVID-19 cases have dropped to low levels in the U.S., but all vaccines are less powerful against newer mutants than earlier versions of the virus - and health officials are warily watching an omicron sibling that’s causing worrisome jumps in infections in other countries.

Pfizer had asked the FDA to clear a fourth shot for people 65 and older, while Moderna requested another dose for all adults “to provide flexibility” for the government to decide who really needs one.

FDA’s Marks said regulators set the age at 50 because that’s when chronic conditions that increase the risks from COVID-19 become more common.

Until now, the FDA had allowed a fourth vaccine dose only for the immune-compromised as young as 12. Vaccines have a harder time revving up severely weak immune systems, and Marks said their protection also tends to wane sooner. Tuesday’s decision allows them another booster, too - a fifth dose. Only the Pfizer vaccine can be used in those as young as 12; Moderna’s is for adults.

What about people who got Johnson & Johnson’s single-dose shot? They already were eligible for one booster of any kind. Of the 1.3 million who got a second J&J shot, the CDC said now they may choose a third dose - either Moderna or Pfizer.