(CNN) — The U.S. Food and Drug Administration has expanded the emergency use authorization of the Pfizer and Moderna COVID-19 vaccines to allow adults age 50 and older to get a second booster as early as four months after their first booster dose of any COVID-19 vaccine.

The move extends the availability of additional boosters to healthy older adults. The FDA had previously allowed second booster shots for anyone 12 years of age or older who was severely immune deficient, starting four months after their first booster.

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The US Centers for Disease Control and Prevention is expected to follow with what’s known as a permissive recommendation — a statement that the shots may be used in this age group for those who want them. The agency is not expected to officially recommend the shots, however.

There is general scientific agreement that third doses help strengthen immunity against severe illness from the virus that causes COVID-19. But the science is far from settled on if or even when fourth doses might be needed since the vaccines continue to offer a high degree of protection against COVID-19 hospitalization and death, even as protection against illness wanes.

Much of the evidence examining the safety and effectiveness of a second booster shot comes from Israel, which has been recommending a fourth dose of coronavirus vaccine to adults age 18 and older since the end of January.

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In a large study of more than half a million adults over the age of 60, those who received a second booster, or fourth dose, of a COVID-19 vaccine had 78% lower odds of death during the Omicron wave compared to those who had a third shot at least four months earlier. But the numbers of deaths were relatively low in both groups. After 40 days of follow-up, there were 232 total deaths out of nearly 234,000 people who’d only had three doses of the Pfizer-BioNTech vaccine, compared with 92 deaths out of 328,000 people.

A smaller study of health care workers, which included younger adults, found that fourth boosters were safe and restored antibodies to the same levels reached after third doses. But fourth doses were only moderately effective — about 30 to 40% — at preventing illness. And most of the workers who got sick still had high viral loads, suggested that they were capable of transmitting the infection to others.

Additional studies from the UK show that the antibody boost from a booster dose wanes very quickly, within a matter of weeks. So some experts feel that considering available resources and the diminishing appetite to continue to get more and more boosters, that the United States should wait until there’s a clear danger from a new wave of infections to roll out fourth doses.

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