By Stephanie Stahl

PHILADELPHIA (CBS) — The U.S. Food and Drug Administration on Tuesday expanded the emergency use authorization for people over 50 years old to get a second COVID-19 booster shot. While there are no safety concerns about a second booster, some experts say there’s not a lot of evidence at this point to show that a fourth shot would be beneficial for healthy people who have already been boosted.

The FDA approved a second COVID-19 booster for people over the age of 50 as early as four months after their first.

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“It’s a bit of a cautious move and something that we’re all kind of wondering why we’re making that decision without data,” said John Wherry.

Wherry is the director of the Penn Institute for Immunology.

The FDA ruled on second boosters without input from its independent advisers, using research from Israel that suggested some benefit from the additional shot.

But it might not be necessary.

“Certainly for a young, healthy mid-50s-age individual, protection from severe disease, hospitalization and death is still very good after three doses,” Wherry said.

He says waiting six to 12 months between boosters provides better immune response and there’s another timing consideration.

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Community transmission rates are very low right now.

“There’s not a lot of risk of getting exposed right now,” Wherry said. “We may want that booster to be timed next time we see the beginning of a wave coming.”

For people over 65, there is evidence a second booster is beneficial now because older people lose protection from the vaccine more rapidly, increasing the risk for severe disease.

For people with a weakened immune system, the FDA has approved a third booster or a fifth shot.

But for people who are healthy, the science is unclear about the need for a second booster at this time.

“Personally, I’m not going to get a fourth dose right now,” Wherry said.

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The FDA says another booster could be available in the fall. It may be targeted to newer variants or a mixture of variants.

Stephanie Stahl