Americans ages 65 and older should receive an additional dose of the latest Covid vaccine this spring, the Centers for Disease Control and Prevention said on Wednesday.

The spring shot would be a second dose of the most recent iteration of the Pfizer-BioNTech, Moderna or Novavax vaccines introduced in the fall. People who are immunocompromised are already eligible for additional doses of the vaccine.

At a meeting of the agency’s Advisory Committee on Immunization Practices earlier on Wednesday, federal researchers presented preliminary data showing that the latest vaccines have an effectiveness of about 40 to 50 percent against symptomatic infection or hospitalization, although estimates against currently circulating variants were based on small numbers.

In October and November, adults who received a fall dose accounted for 4 percent of Covid-related hospitalizations. Those who got a booster in the fall of 2022, but not the updated vaccine this fall, accounted for 25 percent.

Still, a second dose this spring would not be cost-effective for adults 18 to 64 years old, who are at lower risk of severe illness and hospitalization than older adults, according to modeling presented at the meeting.

Older adults and those with weakened immune systems because of illnesses or medications would benefit the most from a spring dose, the advisers concluded.