Who is eligible?

As of now, the booster shots are available to those who received a Pfizer/BioNtech vaccine, and received their second dose at least six months ago. The boosters are approved for those who are 65 and older, as well as those 18 to 64 who are either at high risk of severe COVID because of an underlying medical condition or have jobs or living situations put them at high risk.

What underlying medical conditions put me at high risk of severe COVID?

The CDC has published a complete list of the medical conditions that put adults at high risk of severe COVID. The list includes cancer, dementia, diabetes, obesity, high blood pressure, chronic lung or kidney disease, pregnancy, heart conditions, liver disease, and down syndrome, among others. Others may be added to the list “as the science evolves,” according to the CDC.

The CDC does note that while people ages 18 to 49 with underlying conditions are eligible to receive a booster, they should evaluate their individual benefits and risks before doing so. The agency does not provide the same caution to those over 50 deemed at-risk.

Also, back in August, the FDA green-lit a third dose of both the Pfizer and Moderna vaccines for individuals with compromised immune systems who received their second dose at least four weeks prior. These individuals include solid organ transplant recipients or those with similarly compromised immune systems, such as cancer patients undergoing chemotherapy. Authorization of a third dose for this group came first because these individuals often did not get adequate responses to their first and second COVID vaccine doses as a result of being immunocompromised.

What jobs and living situations qualify as high-risk?

The FDA specifically highlighted a variety of professions and housing situations that frequently put people at high risk of exposure to infected individuals. Health care workers, teachers and day care staff, grocery workers, nursing home residents and those in homeless shelters or prisons, among others, all are eligible to receive a booster, according to an FDA release.

When can I get a booster if I received Moderna or Johnson & Johnson?

Regulatory agencies have not yet offered a recommendation on booster shots for those who received Moderna and Johnson & Johnson, though some experts believe that could happen within the next few weeks.

Moderna, which also makes a two-dose mRNA vaccine for COVID with similar technology to that of Pfizer, has applied to the FDA for emergency authorization of its COVID booster shot. The agency is expected to consider the application in the coming weeks.

Initial data on Johnson & Johnson’s booster vaccine was promising, showing that a booster dose provided 100% protection against severe disease when given two months after the first inoculation (unlike that of Pfizer and Moderna, the J&J vaccine was initially given as a single dose). The company said in a statement last week that it had provided that data to the FDA.

Can I mix vaccines?

Not yet. The National Institutes of Health (NIH) will soon release results on a study on mixing and matching different COVID-19 vaccines, White House chief medical advisor Dr. Anthony Fauci said Tuesday.

Should results of this study show that there are no adverse health effects associated with mixing and matching, it could provide another avenue for those at high risk for serious illness to receive a booster.

More health care and Big Pharma coverage from Fortune:

• Health insurers justify passing COVID treatment costs on to patients despite massive profits
• Why pregnant people are now eligible for COVID-19 vaccine boosters
• Portugal leads the world in COVID-19 vaccinations
• A WHO-approved Novavax vaccine could upstage mRNA jabs—if it can solve its manufacturing delays
• Duckworth, Raimondo: Congress must back the president’s historic investment in home care workers