JNJ-78436735 during Phase 3 trials. JNJ-78436735 is Janssen Pharmaceuticals Companies of Johnson & Johnson's vaccine. It is a one-dose vaccine for individuals 18 and over, with full protection 28 days after vaccination. The vaccine has been shown to be effective at preventing severe COVID-19 illness, hospitalization, and death. (Photo courtesy of Janssen Pharmaceuticals)

The U.S. Food and Drug Administration approved booster COVID-19 vaccine doses Wednesday for people who previously received the Moderna and Johnson & Johnson vaccines as well as the mixing of booster doses, regardless of which vaccine brand a person initially received.

Booster vaccines had previously been authorized for a swath of people — including those age 65 and up and people age 18 to 64 with underlying health conditions — who were originally immunized with the two-dose vaccine developed by Pfizer-BioNTech.

As of Wednesday, eligible vaccinated people can get a booster dose regardless of which vaccine they received first, provided that it has been at least six months since their primary series of the two-dose Moderna and Pfizer vaccines or at least two months after their first Johnson & Johnson vaccination.

In addition, the FDA said it will be safe and effective for people to “mix and match” by receiving a booster dose of a vaccine, such as Moderna, if they received a different vaccine first.

Moderna boosters, according to the FDA’s authorization, will be half the dose administered during the initial two-dose series while Pfizer and Johnson & Johnson boosters will be the same dose size as their initial shots.

“As the pandemic continues to impact the country, science has shown that vaccination continues to be the safest and most effective way to prevent COVID-19, including the most serious consequences of the disease, such as hospitalization and death,” Acting FDA Commissioner Dr. Janet Woodcock said in a statement.

The FDA and officials at the U.S. Centers for Disease Control and Prevention began approving the use of booster vaccines in August — at that time, only for people with weakened or compromised immune systems — as data has shown vaccine-triggered immune responses tend to decline in the months following initial administration.

While the three available vaccines remain highly effective at preventing serious COVID-19 illness and death, public health officials at all levels have argued that preemptively boosting the immune responses of those at particularly high-risk for serious illness, such as immunocompromised people and nursing home residents, will maximize protection against existing and potential variants of the virus, which could become more contagious and even circumvent vaccine protections.

Eventually, some officials have argued, COVID-19 vaccine boosters could become an annual offering similar to annual flu shots rather than one-time administrations.

“The available data suggest waning immunity in some populations who are fully vaccinated,” Woodcock said. “The availability of these authorized boosters is important for continued protection against COVID-19 disease.”

While Woodcock and other federal public health officials have endorsed the idea, regulators have so far shied away from authorizing follow-up vaccine doses for all vaccinated adults, arguing that targeted administration of booster doses is more apt for the pandemic’s current landscape.

Dr. Peter Marks, the director of the FDA’s Center for Biologics Evaluation and Research, said in a statement that boosters for additional demographics remain a possibility.

“We will work to accrue additional data as quickly as possible to further assess the benefits and risks of the use of booster doses in additional populations and plan to update the healthcare community and public with our determination in the coming weeks,” he said.