CDC advisors vote unanimously to give boosters to everyone with a J&J shot and some with Moderna. Here's how to know if you should get one.

• Advisors to the CDC voted Thursday to recommend booster shots for everyone who's had Johnson & Johnson's vaccine.
• The committee also recommended Moderna boosters for adults 65 and up, and said other vulnerable adults may want to get a boost too, depending on their circumstances.

Free booster shots for Moderna and Johnson & Johnson COVID-19 vaccines are here.

An influential advisory committee to the US Centers for Disease Control and Prevention unanimously voted on Thursday to recommend booster shots for some Moderna vaccine recipients and all Johnson & Johnson vaccine recipients.

A final OK from the CDC - allowing booster shots into tens of millions more arms across the country - came just hours later on Thursday evening.

Boosters were far more heartily recommended for everyone who's gotten J&J, whereas for Moderna, the committee emphasized that it's important to weigh the potential risks and benefits of a third shot, especially for younger adults.

How the committee voted

The independent committee voted unanimously Thursday afternoon to recommend a booster shot should be given to:

• Everyone who's gotten J&J's vaccine (at least 2 months after their first shot)

The committee also voted unanimously to recommend a booster shot should be given to:

• People 65 & up who've had Moderna (at least 6 months after their first two shots)
• People ages 50-64 who've had Moderna and have certain underlying medical conditions (at least 6 months after their first two shots)
• People who live in long term care settings like nursing homes and group homes who've had Moderna (at least 6 months after their first two shots)

The committee gave a more mild endorsement of boosters for some other groups of people who've been vaccinated with Moderna (in line with previous Pfizer booster recommendations).

Moderna boosts may be provided (after 6 months) to:

• People ages 18-64 who've had Moderna who live or work somewhere that puts them at increased risk of catching COVID-19
• People ages 18-49 who've had Moderna with underlying conditions

But the doctors, nurses, and other public health experts on the committee were somewhat conflicted about that advice. Whether booster vaccines are really necessary for younger adults who've had Moderna and Pfizer's shots is up for debate.

The definition of who's fully vaccinated isn't going to change, at least for now. (Though there was some discussion about making J&J a two shot vaccine, eventually.)

A new, more flexible approach to boosters

Instead of insisting that people get the exact same brand of shot for their booster, the committee opted for a more flexible approach, letting doctors, pharmacists, and individuals decide which booster to use.

Their decisions came after hours of data presentations from the two pharmaceutical companies, as well as input from independent vaccine researchers. Though not many Moderna and J&J recipients have received boosters yet, the data presented suggests that mix and match boosts are both safe, and work well to amp up the immune protections afforded by the COVID-19 vaccines.

Boosters for J&J recipients are top priority

Boosters are a higher priority for J&J recipients, since many real world studies show that the single shot vaccine is not protecting people as well from infection, hospitalization, or death, as Pfizer and Moderna's two-shot vaccines.

"It appears as if the Moderna vaccine protection has longer legs," Dr. Sarah Long, a pediatrician on the advisory committee said during the meeting, summing up her own takeaways. "There is a real need to protect people who have received Janssen [J&J] vaccine."

(Fewer than one in 10 people vaccinated in the US have had J&J.)

Specific advice for young men and young women

The advisory committee also spent several hours in the afternoon pouring over safety data on vaccines. CDC representatives shared information on the rare, but heightened risk of myocarditis after mRNA vaccines in young men, and the extremely rare, but heightened risk of thrombosis with thrombocytopenia (TTS) in young women after J&J's shot, a condition that has been fatal in a few women.

Committee member Dr. Helen Keipp Talbot from Vanderbilt University said mix and match boosters will be "priceless" for these groups. Young women who originally got a J&J vaccine might opt for an mRNA booster, while young men who got mRNA shots could decide they'd prefer to have J&J.

"We can take the time to be much more thoughtful and careful when we weigh the benefits and risks," she said.

"Those that are not at high risk [of severe COVID-19] should really be thoughtful."

This story has been updated with the CDC's final decision.