(CNN) The US Centers for Disease Control and Prevention has weighed in with the latest word on booster vaccines and drugstores have already started advertising their supply.
Any of the three authorized vaccines in the US -- Pfizer/BioNTech's, Moderna's or the Johnson & Johnson Janssen vaccine -- may be used for a booster dose for eligible people.
But it's not a complete free-for all. Here are some things to know:
There are not any booster police, but the CDC and the US Food and Drug Administration have set some eligibility requirements around who should be seeking booster shots.
People who got a Pfizer or Moderna series of two shots six months ago or longer may consider getting a booster if they are also:
• 65 or older.
• At risk of severe Covid-19 from a breakthrough infection because of a medical condition such as diabetes, kidney disease or pregnancy.
• At risk because of living conditions or work, so people who work in healthcare, frontline healthcare workers, people living in shelters or incarcerated people.
Moderna's vaccine is given as a half dose for boosters; Pfizer's is given as a full dose.
Anyone who got a Johnson & Johnson shot two months ago or longer is eligible two months after getting the first shot. That's because the vaccine doesn't provide as much protection as the other two do, and studies show a booster brings protection up to levels of more than 90%.
Federal health officials made a point of urging pregnant women to get a booster.
"If you are eligible for a boost and you're pregnant, you should also get your boost during that period of time. And I would say for nursing (mothers) as well," CDC Director Dr. Rochelle Walensky told a news conference Friday.
And there's no reason to wait if you are eligible. The booster can be administered along with other shots, such as flu shots or shingles, pneumococcal or other vaccines.
For the most part, it doesn't matter. Studies indicate it's all right to mix and match vaccine doses and in some cases it may provide a more powerful boost to get a different vaccine type, but any booster dose will bring immune protection up to very high levels.
And while some of the initial data suggest that boosting with a different vaccine type from your initial series might provide a stronger response, it's only preliminary data. All the groups in an ongoing study by the National Institutes of Health that compares the three authorized vaccines got a big jump in immunity from a booster shot, starting about 15 days after the booster.
Some people might want to use a different vaccine type. For example, a younger man who initially got a Moderna vaccine might seek J&J for the boost, because the mRNA vaccines like Pfizer's and Moderna's are linked with a slight risk of a heart inflammation called myocarditis, while J&J's isn't. Younger men -- those under 40 -- are at higher risk of myocarditis.
And a woman under 50 might prefer to get an mRNA vaccine for her boost, because the J&J vaccine is linked with a slightly higher risk of a rare blood clotting condition that is more likely to affect younger women.
"As a woman in this age group, I opted to get the Pfizer vaccine," said Dr. Leana Wen, a former Baltimore city health commissioner who's a CNN medical analyst.
"I didn't choose the mRNA vaccine because it's necessarily more effective, but rather because I wanted to avoid the potential of the very serious side effect associated with J&J in younger women. I chose Pfizer purely out of convenience, because the Moderna 50 microgram booster was not yet available when I decided to get my booster. I wouldn't have a reason to prefer Pfizer over Moderna; I would have received whatever I had access to."
Anyone who had a severe reaction to one vaccine type should probably check with their doctor first and should consider getting a different vaccine type for their booster.
The CDC assumes most people will just get the same vaccine as a booster as they got originally, and says that would be fine.
The data indicate that the vaccines are still providing very strong protection for most people. While there's some evidence of waning immunity for some people -- especially older people -- they are still strongly protecting most people against severe disease, hospitalization and death.
The CDC is expected to release some more detailed guidelines about boosters and, at Thursday's meeting of vaccine advisers, laid out clinical considerations for who should consider getting boosters. These might include people who are caregivers of people who are at high risk of severe disease. They might want to get boosted so they are even more strongly protected against mild or asymptomatic infections, to protect the people they are caring for.
Federal health officials say they are watching data carefully to see if there are indications that immunity is waning for younger groups of healthy people. If so, they say, they can move quickly to lower the age range and other eligibility criteria for boosters.
Just as with the first round of vaccines, Covid-19 booster shots are free. "No insurance or ID required," Jeff Zients, White House Coronavirus Response Coordinator, told a news conference Friday.
Some providers may ask for health insurance information because they will bill private insurers for costs associated with providing vaccines. Covid-19 vaccines are free regardless of immigration status.
The federal government has a website, vaccines.gov, that allows a fairly detailed search. You can narrow the search by vaccine brand and by location.
"In fact, boosters are available at over 80,000 locations across the country, including more than 40,000 local pharmacies, as well as thousands of doctors' offices, community health centers, rural health clinics, and community-based vaccination sites. Ninety percent of Americans live within five miles of a vaccine site," Zients said.