Updated March 29, 2022 at 4:21 PM ET

The Centers for Disease Control and prevention is now recommending that certain immunocompromised individuals and people over the age of 50 who received an initial Moderna or Pfizer-BioNTech booster dose at least four months ago are now eligible for another shot of either vaccine.

The public health agency also said adults who received a primary vaccine and booster dose of Johnson & Johnson's Janssen COVID-19 vaccine at least four months ago may now receive a second booster dose of either the Moderna or Pfizer-BioNTech COVID-19 vaccine.

The moves follow the Food and Drug Administration's authorization of the second booster doses earlier Tuesday.

"These updated recommendations acknowledge the increased risk of severe disease in certain populations including those who are elderly or over the age of 50 with multiple underlying conditions, along with the currently available data on vaccine and booster effectiveness," the agency said in a statement.

The recommendation comes as BA.2, an even more contagious version of the omicron variant, continues to spread in the U.S., and concern mounts that it could fuel another surge. BA.2 is now the dominant strain in the U.S., making up 54.9% of cases, according to the CDC.

"Current evidence suggests some waning of protection over time against serious outcomes from COVID-19 in older and immunocompromised individuals. Based on an analysis of emerging data, a second booster dose of either the Pfizer-BioNTech or Moderna COVID-19 vaccine could help increase protection levels for these higher-risk individuals," said Peter Marks, M.D., Ph.D., director of the FDA's Center for Biologics Evaluation and Research.

"Additionally, the data show that an initial booster dose is critical in helping to protect all adults from the potentially severe outcomes of COVID-19. So, those who have not received their initial booster dose are strongly encouraged to do so," Marks said.

Dr. Eric Topol, founder and director of the Scripps Research Translational Institute, supported the FDA's authorization, saying there are many people who are at least four to six months past their third shot.

"Without protection against the omicron variant, particularly now we're confronting BA.2, there's a very high risk of hospitalization and death," he said.

But others question the plan. The vaccines are still doing a good job of protecting people from getting seriously ill. Critics say there just isn't enough evidence yet that another shot is needed and that it would provide stronger protection that would last.

"From a scientific perspective, we still don't have definitive evidence that giving a second booster dose is the right way to go in older people," said Dr. Celine Gounder, an infectious disease specialist and a senior fellow and editor at Kaiser Health News.

She said data out of Israel shows an additional booster dose does reduce the risk of severe disease, hospitalization and death for people over the age of 60. But she pointed out it's unclear how long that extra protection actually lasts.

"I don't think it hurts," Dr. Carlos del Rio, an infectious disease researcher at Emory University told NPR. "But the reality is the benefit against infection will be short lived and thus likely of little benefit for most people." He also cited the Israeli data showing benefits for those 60 and older.

Administration officials said it's important to give people the option of a second booster as quickly as possible. The plan to offer it to people younger than 60 was made to ensure that more vulnerable people, particularly people of color who are more likely to suffer other health problems that put them at risk, also have the option of an additional booster.

But other infectious disease specialists said the administration should be focusing on getting people their primary doses and first boosters.

"What concerns me is that we are not investing in increasing the coverage of booster doses and even the primary doses," said Dr. Saad Omer, the director of the Yale Institute for Global Health. "These are the things that are not receiving enough attention."

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