People ages 50 and older can now receive a second booster dose of the Pfizer-BioNTech or the Moderna Covid-19 vaccine.
Hours after the Food and Drug Administration authorized the second booster on Tuesday, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, issued a statement allowing shots to begin immediately.
“Boosters are safe, and people over the age of 50 can now get an additional booster 4 months after their prior dose to increase their protection further,” Walensky said.
Full coverage of the Covid-19 pandemic
The FDA had already authorized a fourth shot for immunocompromised individuals. Also Tuesday, the FDA cleared a fifth shot, or second booster, for that group. Previously, immunocompromised individuals were authorized to receive a three-dose primary series of a vaccine followed by a booster.
The speed with which the two agencies cleared the additional vaccine doses indicates their concern about the spread of the extremely contagious omicron subvariant, known as BA.2. The subvariant accounts for about 55 percent of the new cases in the country, according to the latest data from the CDC.
About a third of people ages 50 to 65 have significant underlying conditions that put them at risk of severe Covid, Dr. Peter Marks, the FDA’s top vaccine regulator, said during a briefing Tuesday.
"If it were my relatives, I would be sending them out to do this again because of the higher level of protection," Marks said.
He added that it's possible people may need to get yet another dose of a Covid vaccine this fall.
That later dose, Marks said, may not be the same version of the shots currently in use — regulators may transition to a variant-specific vaccine or one that targets more than one strain. Both Pfizer and Moderna are testing a vaccine that targets the omicron variant. Moderna is also testing a shot that targets both the delta and omicron variants.
"At some point, we are going to have to realize that this is a virus that's going to be with us and that we have to come to grips with dealing with it on a regular basis," he said.
The share of people who've received their first booster remains low in the United States, with fewer than 50 percent of those who are eligible for a booster having received one, according to data from the CDC.
The FDA appeared to rely on data from Israel, where officials began offering fourth doses in December to certain vulnerable groups.
Officials may have a hard time persuading people to get a second booster — as Covid cases, hospitalizations and deaths remain low, some people feel a lack of urgency and have a reduced fear of the disease, experts say.
In addition, many say the U.S. mishandled the rollout of the first booster shot last year when it authorized the dose in a confusing manner.
Both Pfizer and Moderna asked the FDA this month to authorize a second booster, arguing that an additional booster is now needed because research shows protection from the initial booster wanes after a few months. Pfizer's request was limited to adults ages 65 and older; Moderna's was for all adults.
Among the research cited is a study from the CDC, which found that the effectiveness of Pfizer’s and Moderna’s vaccines against hospitalization decreased from 91 percent two months after the initial booster shot to 78 percent after four months. The study included the months of December and January, when the omicron variant was spreading rapidly throughout the U.S.
Marks on Tuesday also cited data from Israel that showed people 60 and up who received a second booster after at least four months from the initial booster shot had a lower death rate, compared to those 60 and up who only received one booster dose.
Experts are divided on whether an additional dose is necessary right now, noting that while protection against infection is relatively low, two doses and a booster still provide a high amount of protection against severe disease, hospitalization and deaths, especially in young, healthy people.
However, many experts noted that a second booster may be needed for certain groups, such as older adults or those with underlying medical conditions, in the future.
In addition to Israel, other countries, including Chile and Sweden, also allow for a fourth vaccine dose for certain vulnerable populations.
John Moore, a professor of microbiology and immunology at Weill Cornell Medical College, said the FDA's decision to allow second boosters for people as young as 50 "seems excessively cautious" as there isn't much data to show people under 65 would benefit from the additional shot.
He also questioned the timing, noting that Covid cases and hospitalizations remain at low levels and there's no indication yet that there will be a surge in cases during the summer months.
"What would then happen if there’s a winter surge? Moore asked. "Would people then be expected to get Dose 5 in the fall? Is repetitive boosting now going to be a national policy?"
Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia and a member of the FDA's advisory committee, also questioned the agency's reasoning for including the younger age group.
He also said he worries about the possibility of "immune exhaustion" from frequent boosters. That's when the immune system doesn't mount the same kind of antibody response it did with previous shots.
The second boosters use the same formulation and dosage as the initial boosters for adults — 30 micrograms for Pfizer's and 50 micrograms for Moderna's.
Both companies said the additional boosters were generally well tolerated by people participating in the trials.
Tuesday’s decision bypassed the independent panel of experts from both the FDA and the CDC, which have issued recommendations to the agencies throughout the pandemic on whether they should clear additional shots, and if so, for which groups.
Follow NBC HEALTH on Twitter & Facebook.