The US Food and Drug Administration is making it a high priority, the official said.
Second boosters have been approved for adults 50 and older, as well as for some people with weakened immune systems, since the end of March. But younger adults are only eligible for one booster shot, which was authorized in November. Federal agencies are looking to quickly authorize a second booster for all adults, the source said. Some experts are concerned that the immunity of younger adults may be waning as Covid-19 cases rise with the dominance of the BA.5 Omicron sub-variant. Reinfections are more likely with BA.5 than with all previous variants because of the immune defense characteristics, Dr. Eric Topol, a cardiologist and professor of molecular medicine at Scripps Research, told CNNI on Monday.
The US Centers for Disease Control and Prevention reports that the BA.4 and BA.5 subtypes account for more than 70% of new infections in the country. But while these sub-variants may partially escape the immunity created by the vaccine and by previous infection, vaccination still likely protects against severe disease.
It remains unclear how many American adults would receive a second booster dose if one were allowed. As of Thursday, about half of Americans 18 and older who are eligible for their first booster have received it, according to the CDC. Just over a quarter of eligible adults 50 and older received a second booster.
A study published in May found that a fourth dose of Moderna’s or Pfizer/BioNTech’s Covid-19 mRNA vaccine provided a “substantial” boost in immunity at similar or even better levels than a third dose. The study, which was published in the medical journal The Lancet and involved participants whose average age was 70.1, also showed that some people who had higher levels of antibodies before the fourth dose of the Covid-19 vaccine had only “limited” reinforcement.
Those with a history of Covid-19 infection had a similarly limited response. The authors say this suggests there may be a ceiling or maximum response that can come with a fourth dose of vaccine.
Two earlier studies from Israel, conducted among participants aged 60 or older, showed that hospitalization and mortality from Covid-19 could be reduced with a fourth dose of vaccine given at least four months after the third dose. The reduction in hospitalizations and deaths is maintained over time with this fourth injection.
This story was updated with additional information Monday.
CNN’s Jen Christensen contributed to this report.
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