Jatniel Hernandez fills syringes with Covid-19 vaccine booster shots at a Covid-19 vaccination clinic on April 06, 2022, in San Rafael, California.
CNN  — 

US health officials are urgently working on a plan to allow second Covid-19 boosters for all adults, a senior White House official confirmed to CNN on Monday.

The US Food and Drug Administration is making it a high priority, the official said.

Second boosters have been authorized for adults 50 and older, as well as some people with weakened immune systems, since late March. But younger adults are eligible for only one booster shot, which was authorized in November. Federal agencies are looking to move quickly on authorizing a second booster for all adults, the source said.

Some experts are concerned that younger adults’ immunity may be waning as Covid-19 cases rise with the dominance of the BA.5 Omicron subvariant. Reinfections are more likely with BA.5 than with any previous variants because of immune escape features, Dr. Eric Topol, a cardiologist and professor of molecular medicine at Scripps Research, said on CNNI Monday.

The US Centers for Disease Control and Prevention reports that the BA.4 and BA.5 subvariants constitute more than 70% of new infections in the country. But while these subvariants may partially escape the immunity produced by the vaccine and by prior infection, vaccination still likely protects against severe illness.

It remains unclear how many American adults would get a second booster dose if one is authorized. As of Thursday, about half of Americans 18 and up who were eligible for a first booster had gotten it, according to the CDC. Just over a quarter of eligible adults 50 and over have gotten a second booster.

A study published in May found that a fourth dose of Moderna or Pfizer/BioNTech’s mRNA Covid-19 vaccine provides a “substantial” boost to immunity at similar or even better levels than a third dose.

The study, which was published in the medical journal The Lancet and included participants whose median age was 70.1 years, also showed that some people who had higher levels of antibodies before the fourth dose of the Covid-19 vaccine had only “limited” boosting.

Those with a history of Covid-19 infection had a similarly limited response. The authors say this suggests that there may be a ceiling or maximum response that can come with a fourth vaccine dose.

Two earlier studies out of Israel, conducted among participants age 60 or older, showed that hospitalization and death rates from Covid-19 could be reduced with a fourth vaccine dose given at least four months after the third dose. The reduction in hospitalizations and death persisted over time with this fourth shot.

This story has been updated with additional information Monday.

CNN’s Jen Christensen contributed to this report.