Writing for the New York Times, Tara Parker-Pope answers eight key questions about Covid-19 booster shots following the FDA authorization and the CDC recommendation released last week.

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1. Who is eligible for booster shots?

Pfizer-BioNTech vaccine recipients ages 65 and older and those who live in long-term care facilities are eligible for booster shots, Parker-Pope reports. Adults with underlying medical conditions that put them at risk for severe Covid-19, as well as workers who are at increased risk due to institutional or occupational exposure, are also eligible for booster shots.

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(According to Parker-Pope, people with weakened immune systems who received Pfizer-BioNTech's vaccine or Moderna's vaccine are also eligible for a third dose four weeks following their second shot, but she notes that this is not so much a booster dose as it is "part of the recommended immunization schedule for those with compromised immune systems who don't generate a robust response after just two shots.")

2. What underlying medical conditions qualify you for booster shots?

According to CDC, several medical conditions may increase an individual's risk of complications from Covid-19, including:

• Cancer
• Chronic lung or kidney disease
• Dementia and certain disabilities
• Diabetes
• Heart disease
• High blood pressure
• Obesity

In its recommendation last week, CDC said individuals over 50 with underlying medical conditions should receive booster shots. However, the agency recommended that people ages 18 to 49 with underlying medical conditions determine their own individual risk before deciding to get a booster.

3. Which professions are eligible for booster shots?

FDA authorized booster shots for people whose jobs include high risk of exposure to coronavirus infection, Parker-Pope reports. Currently, health care workers, teachers and day care staff, grocery store workers, and staff at homeless shelters or prisons are eligible for booster shots.

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4. Will booster shots be made available to the general public?

Although the Biden administration's initial booster plan was intended for all Americans, FDA and CDC have so far recommended boosters only for select populations. However, according to Parker-Pope, the availability of booster shots may be expanded in the future as more data on the durability of vaccine antibodies over time becomes available.

Overall, the scientific community agrees that all Covid-19 vaccines continue to be highly protective against severe illness, hospitalization, and death, Parker-Pope reports.

5. If you're eligible, how and when can you get a booster shot?

Eligible individuals can get a booster shot six months after their second dose of the Pfizer-BioNTech vaccine. Currently, there is no safety data on receiving booster doses at an earlier interval, and getting a booster too soon may not increase antibodies meaningfully. (And again, Parker-Pope reports that people who are severely immune compromised can get a third shot sooner as part of their recommended immunization schedule—not as a "booster.")

According to Parker-Pope, health departments, pharmacies, and doctor's offices that have already been administering first and second doses will likely administer booster shots in a similar fashion.

Parker-Pope recommends people call ahead to schedule an appointment and bring their vaccine cards when they go. People with underlying medical conditions may also want to discuss the risks and benefits of getting a booster shot with their providers before they ultimately decide.

6. What side effects do booster shots have?

Data on the side effects of booster shots has been limited, Parker-Pope reports, but so far, reports from Pfizer and Moderna have indicated any side effects are similar to those experienced after two vaccine doses. According to CDC, the most commonly reported side effects were fatigue and pain at the injection site, with most symptoms being mild to moderate.

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Separately, a survey from Israel, where people are already receiving booster doses, found that 88% of Pfizer vaccine recipients who received a booster dose said they felt "similar or better" to how they felt after their second dose. Around 33% reported some side effects, with the most common being pain at the injection site, and 1% said they sought medical care for at least one side effect.

7. Are people who received different vaccines eligible for booster shots?

So far, federal regulators have not made recommendations regarding booster shots for those who received the Moderna or Johnson & Johnson (J&J) vaccine, although officials have said recommendations for these groups may come soon, Parker-Pope reports.

Currently, Moderna, whose vaccine uses similar technology to Pfizer-BioNTech's, has applied for FDA authorization of its booster shots. J&J has not submitted an application to FDA for booster shots of its vaccine, which is a single dose and uses a different method to produce antibodies. However, the company last week announced data showing two doses of its vaccine increased efficacy against mild to severe Covid-19 from 74% to 94% in the United States.

8. Can you get a booster of a different vaccine?

Regulators have not recommended people mix two different Covid-19 vaccines, Parker-Pope reports. For now, people who received Pfizer-BioNTech's vaccine are recommended to get a booster of the same vaccine, while recipients of Moderna's and J&J's vaccines should wait until boosters of those specific vaccines have been approved.

However, during CDC's Advisory Committee on Immunization Practices (ACIP) meeting last week, many members seemed to be in favor of mixing vaccine doses, MedPage Today reports. Wilbur Chen, an ACIP member and physician from the University of Maryland, said boosters should be recommended "agnostic of primary series."

Separately, Leana Wen, a professor of health policy and management at George Washington University, has advocated for using mRNA vaccines, such as Pfizer-BioNTech and Moderna, as boosters for all J&J recipients. She cited data from Europe that showed mixing the AstraZeneca vaccine, which uses technology similar to J&J, and mRNA vaccines was safe and effective.

According to MedPage Today, more definitive information about mixing different vaccines will be available in the future. The National Institutes of Allergy and Infectious Diseases will have booster data from all three vaccines available for analysis later in the fall, and NIH is currently conducting a specific trial to test mixing different vaccines. (Parker-Pope, New York Times, 9/24; Fiore, MedPage Today, 9/23)