The U.S. Food and Drug Administration said Tuesday it has approved the emergency use of an additional dose of coronavirus vaccines separately developed by Pfizer Inc. and Moderna Inc., with people aged 50 or older eligible for the fourth shots.

The regulators decided to greenlight the "second booster" for people in the age bracket after finding the fourth shot appears to have been effective in preventing infected people in Israel from developing serious illness, while there are no fresh safety concerns from additional shots.

People aged 50 or older who received the booster shot more than four months ago will be eligible for a fourth shot.

Additionally, those aged 12 or older who are immunocompromised due to certain types of health issues are also entitled to receive another Pfizer booster, while such individuals aged 18 or older can receive a second Moderna booster.

"Emerging evidence suggests that a second booster dose of an mRNA COVID-19 vaccine improves protection against severe COVID-19 and is not associated with new safety concerns," the FDA said in a statement.

"The agency will continue to evaluate data and information as it becomes available when considering the potential use of a second booster dose in other age groups," the statement added.

According to the Centers for Disease Control and Prevention, the Omicron variant of the novel coronavirus has been waning in the United States.

At the same time, the BA.2 Omicron subvariant, which is believed to be more transmissible than the original strain, has become dominant in new infection cases recently in the country, rekindling concerns of another possible outbreak.