The United States is expanding its COVID-19 booster, requiring 16- and 17-year-olds to receive a third dose of Pfizer’s vaccine.

The United States and many other countries are already urging adults to get booster shots to increase immunity that may weaken within months of vaccination, and this call is even stronger with the discovery of worrying new omicron variants.

On Thursday, the US Food and Drug Administration urgently authorized 16- and 17-year-olds to receive a third dose of the vaccine produced by Pfizer and its partner BioNTech-if six months have passed since the last injection.

One more step: The Centers for Disease Control and Prevention must formally recommend boosters for this age group, and a decision is expected to be made soon.

The Pfizer vaccine is the only option for anyone under the age of 18 in the United States, whether it is an initial vaccination or as a booster vaccine. It is not clear whether or when adolescents under 16 may need a third dose of Pfizer.

Vaccinations for 5-year-old children only started last month, using a special low-dose Pfizer vaccine. By this week, about 5 million children between the ages of 5 and 11 had received their first dose of the vaccine.

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In the United States and most parts of the world, the highly contagious delta variant causes almost all COVID-19 infections. It is not yet clear how the vaccine will resist the new and significantly different omicron mutants. But there is strong evidence that boosters provide a leap in protection against infections caused by the delta, which is currently the biggest threat.

The decision to extend the booster to 16- and 17-year-olds is complicated by the fact that Pfizer injections — and similar vaccines made by Moderna — are associated with rare side effects. It is called myocarditis, and it is an inflammation of the heart mainly seen in young men and teenage boys.

Israeli health officials have provided boosters for teenagers, and they say that side effects from the third dose are still rare.

A study in the United States this week provided additional assurance. Researchers from the National Children’s Hospital checked the medical records and found that this rare side effect is usually mild, and people recover quickly, and COVID-19 itself can cause more serious heart inflammation. The research was published in the journal Circulation on Monday.