Federal health officials have greatly expanded the number of Americans eligible for booster shots, approving Pfizer and Moderna booster doses for anyone 18 and older . And people 18 and up who received the Johnson & Johnson vaccine have already been eligible for a booster dose which basically means most fully vaccinated adults qualify for an additional dose at this point.

So do you have questions about why boosters are being recommended and what the process is like? Heres what you need to know:

1. Is anyone NOT eligible for boosters now?

If youre 18 or older and youre fully vaccinated, youre eligible to be boosted if its been at least six months since your second dose of an mRNA vaccine or two months since you received the Johnson & Johnson shot.

This week, the FDA and the Centers for Disease Control and Prevention OKd booster doses of the Pfizer and Moderna vaccines for anyone 18 and older after that six-month stretch. Previously, only people who were older than 65, who had certain underlying health conditions or who lived or worked in high-risk settings qualified.

The booster for the one-dose Johnson & Johnson vaccine had already been approved for those 18 and older, as long as it had been a two-month stretch.

2. Should you mix manufacturers?

Federal health officials have said its OK to mix-and-match vaccines, and its pretty widely recommended at this point . Most experts say getting boosted with one of the two mRNA vaccines is really your best bet regardless of what you got when you initially you rolled up your sleeves.

The mRNA vaccine you go for doesnt seem to matter much. Its not clear whether there are any benefits to getting boosted with Moderna if you initially had the Pfizer shots, or vice versa. Right now, it really comes down to personal preference and availability though doctors and researchers are learning about boosting in real-time.

That said, youll still get benefits from getting another dose of the Johnson & Johnson vaccine if thats what you went with the first time around. According to the company , a booster dose of its vaccine provides 94% protection against moderate to severe COVID and can increase initial antibody levels by four to six times .

3. Whats the difference between a booster and a third dose? Are they the same?

Although the terms are often used interchangeably in discussions about Pfizer and Moderna shots the two mRNA vaccines booster doses and third doses are not exactly the same . Boosters are given to people who have already had a full course of the vaccine and developed a good response; third (or additional) doses are given to people who already had a full course of the vaccine and did not develop a sufficient immune response, like immunocompromised individuals.

That is why federal regulators approved a third dose for people who are seriously immune-compromised meaning theyre undergoing cancer treatments, for example, or have HIV well before they approved booster shots for people in other high-risk groups.

4. Will additional shots reduce breakthrough infections?

Yes. The decision to make another shot available to Americans is based on available data that makes it clear that protection against COVID-19 decreases over time. Its not clear how common breakthrough infections are at this point because federal health officials really only track cases that end in hospitalization or death, which remains rare. But breakthrough cases can and do happen, so the goal with boosting is both to prevent them and to help make sure that when they do happen, the outcome is relatively mild.

The vaccine is still really protective against severe disease, Adam Ratner , chief of the division of pediatric infectious diseases at Hassenfeld Childrens Hospital at NYU Langone Health, previously told HuffPost.

Fully vaccinated folks, in general, are not the ones who are ending up in the hospital, ending up in the intensive care units, he added. The idea behind boosting is to make sure that we dont get to that point.