Moderna and Johnson & Johnson booster shots received emergency use authorization from the FDA on Wednesday, clearing a crucial regulatory obstacle for millions of Americans wishing to improve their COVID-19 protection. Adults can now “mix and match” their booster injections, according to the FDA, and are no longer required to acquire a second dose from the same brand that vaccinated them the first time.

Many adults who were vaccinated at least six months ago will be eligible for a third Moderna dose, according to the FDA. A second dosage of Johnson & Johnson’s COVID-19 vaccination is available to all 15 million adult recipients. Before the booster injections may be given, vaccine providers must wait for the CDC to formally endorse their use.

According to the CDC, more than 9 million Americans have previously received a booster shot from Pfizer. Only about 1.7 million people have received the third dosage of Moderna’s vaccine, which is presently only available to people who are immunocompromised in the United States.

Commissioner Walensky expanded the age group eligible for Pfizer’s booster shots to include individuals who were at higher risk. The vaccine’s protection against COVID-19 appears to have deteriorated, raising the risk of new variations. Officials from the FDA stated on Wednesday that the agency is still debating whether or not to extend eligibility to younger adults.