LOS ANGELES, April 18 (Xinhua) -- The U.S. Food and Drug Administration (FDA) on Tuesday amended the emergency use authorizations of the Moderna and Pfizer-BioNTech COVID-19 bivalent mRNA vaccines, approving a second dose of the updated COVID-19 booster for older adults and people with weakened immune systems.

Those 65 and older can get a second dose of the updated versions of the Moderna and Pfizer-BioNTech booster at least four months following their initial bivalent dose, the FDA said in a statement.

Most people who are immunocompromised can get an additional updated booster dose at least two months following their last dose, according to the agency.

Most unvaccinated individuals may receive a single dose of a bivalent vaccine, rather than multiple doses of the original monovalent mRNA vaccines.

The monovalent Moderna and Pfizer-BioNTech COVID-19 vaccines are no longer authorized for use in the United States, said the FDA.