Thursday’s recommendations by the U.S. Centers for Disease Control and Prevention on booster doses of Moderna and Johnson & Johnson vaccines opens the door for millions of Americans to receive another layer of protection against COVID-19.

The CDC already approved boosters of the Pfizer vaccine back in September and 11 million additional doses have been administered, including more than 70,000 here in Maine.

But although boosters have now been approved for all three COVID-19 vaccines in circulation, there is still the potential for confusion. So here is a breakdown of where we are.

Q: Am I eligible for a booster?

A: Per the CDC’s updated recommendations, people who received an original two-dose regimen of Pfizer or Moderna vaccine at least six months ago should get a booster if they fall into one of these categories:

• 65 and older.

• 18 and older and have other serious medical conditions.

• 18 and older and live in a long-term care setting.

• 18 and older and work in a high-risk setting, such as a hospital or school.

For those who received the one-dose Johnson & Johnson vaccine, a booster is recommended for everyone at least two months after their original doses.

“Boosters are important and, if eligible, you should get one. We at (the Maine CDC) are focused not just on ensuring booster access, but also on providing first doses,” agency director Dr. Nirav Shah tweeted Friday.

Q: Why are boosters needed anyway?

A: The effectiveness of most vaccines wane over time. Experts have been diligently studying the efficacy of the COVID-19 vaccines for months to arrive at the current recommendations.

The need for boosters does not mean the original vaccines didn’t work. All the data suggests that the vaccines have dramatically reduced the risk of contracting the virus and, more importantly, of being hospitalized or dying from COVID-19.

Boosters have long been used in routine vaccinations for children against illnesses such as chickenpox, mumps, measles and rubella, and for adults, too, against things like tetanus.

It’s a way to help the body create more memory cells to fight off a specific illness.

There is also a distinction between the terms “booster” and “additional dose,” even though they are often used interchangeably.

Additional doses are for older or immunocompromised individuals who may not have gotten the right level of protection from their original vaccine, so they get another shot of the same vaccine.

Boosters are given when an original vaccine’s effectiveness starts to wane, and sometimes the doses are altered to protect against new strains, such as the delta variant, which is responsible for the summer surge in cases across the country that is still being felt in many places, including Maine.

Q: Should I get the booster that matches my original COVID-19 vaccine?

A: All things being equal, yes.

“If you’ve gotten Pfizer or Moderna and if you did well with your primary series, I think it’s quite reasonable to stick with what you’ve got originally,” U.S. Surgeon General Dr. Vivek Murthy said Friday on CNBC.

Having said that, individuals can safely mix and match. If someone got the Pfizer or Moderna vaccine originally, they can get a booster of the other, or of the Johnson & Johnson version. Similarly, if someone received the J&J vaccine, they can opt for a booster of the Pfizer or Moderna vaccine.

Pfizer’s and Johnson & Johnson’s booster vaccines are both full doses. Moderna’s booster is a half dose of the original regimen.

Q: Are there benefits to mixing and matching?

A: In some cases, mixing vaccines may offer better protection, experts said. Early data has shown that following up a first dose of Johnson & Johnson, a more traditional vaccine, with an mRNA vaccine – Moderna or Pfizer – provided significantly higher levels of neutralizing antibodies.

“The data show that folks who got Johnson & Johnson and got boosted with Moderna or with Pfizer had a really strong antibody response,” Murthy said.

But data also showed a strong immune response from a J&J booster dose.

The biggest benefit to mixing and matching is that it gives people options to get their booster as soon as possible. Not every pharmacy or vaccine location will have each vaccine on hand at all times.

Q: What is the difference between the vaccines again?

A: The Johnson & Johnson vaccine is a traditional carrier vaccine. It involves injecting individuals with an inactive adenovirus, or common cold virus, that carries the genetic code of the coronavirus spike proteins. The body’s immune system is then trained to create antibodies and memory cells that protect against the actual virus.

The Pfizer and Moderna vaccines were developed using messenger RNA technology. That involves delivering a piece of genetic code from the coronavirus to host cells in the body. The body then makes copies of the virus’s spike proteins, which stimulates the immune system to produce antibodies and develop memory cells.

Q: Can I expect any side effects?

A: According to data that has been reviewed by the U.S. CDC, individuals have reported minor side effects after receiving a third shot of the Moderna or Pfizer vaccine. They include: pain at the injection site, fatigue, muscle pain, headache and fever, but are often not long-lasting.

The data suggested the side effect rates were similar to those seen after the second dose of an mRNA vaccine.

Data was more limited for the J&J vaccine, but some people reported fever, fatigue and headache after receiving a second dose of that vaccine, the CDC said.

Q: Where can I get a booster?

A: Individuals can consult with their primary care doctor if they wish, who might then make their own recommendation.

Otherwise, boosters will be available at most of the places COVID-19 vaccines have been offered for months. Retail pharmacies, such as Walgreens and Rite Aid, are likely to be the most common places, but some health care practitioners also will be able to provide doses to their patients. Depending on supplies, locations might run out of specific vaccines on any given day, but supply challenges are less problematic than they were at the beginning of the vaccine rollout.

Still, people who are set on a specific brand of vaccine would do well to check before they arrive for their booster.

The Maine CDC has a list of sites that are currently offering additional doses of Pfizer vaccine at: https://www.maine.gov/covid19/vaccines/vaccination-sites. That will be updated soon to include locations offering the other boosters.

Northern Light Health, one of the state’s largest health care providers, said Friday that it plans to open online registration for Moderna and Johnson & Johnson boosters beginning Monday with availability at its clinic beginning Thursday. The site is: https://covid.northernlighthealth.org/. You don’t need to be a Northern Light patient to get a booster at any of its clinics.

Q: Do I need to bring my vaccination card to get the booster?

A: Yes, it’s advisable to bring proof of original vaccination with you when you get a booster, so the practitioner can update the card with that information. Some public venues already require proof of vaccination and it’s possible that could be updated to include boosters at some point. As of right now, though, a booster is not needed to be considered “fully vaccinated.”