The Centers for Disease Control and Prevention opened the floodgates Friday for potentially millions of Americans to get a third COVID-19 shot based on their age, medical conditions or risk on the job.

Confused about whether you’re one of them and whether you should get a booster? Read on.

Who can get a third dose now?

Previously, only people who have specific conditions that impair their immune systems were eligible. Now, you can get a third shot if you previously had the Pfizer vaccine, got your second dose at least six months ago, and either are over 65, have a chronic health condition, or work in a job where you’re at risk of exposure to the virus.

Health conditions that qualify a person for a booster include:

• Cancer
• Chronic kidney disease
• Chronic lung disease, including moderate or severe asthma
• Dementia
• Diabetes
• Down syndrome
• Heart conditions
• HIV
• Weakened immune system
• Liver disease
• Overweight or obesity
• Pregnancy
• Sickle cell disease or thalassemia
• Current or former smoking
• Organ or stem cell transplants
• Stroke or cerebrovascular disease
• Substance use disorder (addiction)

Workplaces that could be considered high-risk include health care facilities, prisons and homeless shelters, according to CDC Director Dr. Rochelle Walensky. The guidance is sufficiently vague that almost all frontline workers could qualify, though.

In Colorado, vaccine providers were told to take people’s word that they qualified for a third shot because of a condition affecting the immune system, and it’s likely they’ll rely on the honor system again.

What if I got the Moderna or Johnson & Johnson shot?

In early September, Moderna announced it had submitted data supporting its application to allow a wider population to receive boosters. Johnson & Johnson also reported this week that a second shot gave recipients more protection.

The U.S. Food and Drug Administration is expected to take up the Moderna and Johnson & Johnson booster question in a few weeks. The FDA doesn’t recommend mixing shot types at this point, because there’s relatively little data, though what information exists hasn’t raised red flags.

Why do I need a booster?

Depending on your age and health, you might not. Scientists are still debating whether boosters have any value for generally healthy people.

There’s consensus that people with compromised immune systems benefit from a third shot, giving them a chance to form defenses that they may not have had before. People who have compromised immune systems are about 400 times as likely to be hospitalized for COVID-19 after vaccination as vaccinated people without a major health condition.

The CDC recommends that people who are over 65, or are over 50 and have a chronic health condition, should get a third shot. The recommendation is weaker for younger people who have health conditions or are at risk of exposure at work, who “may” get the shot if they choose.

The existing shot regimen still offers strong protection against hospitalization and death for healthy people, leading some experts to conclude boosters aren’t necessary for that group. Others argued that the disruption from health care staff and other frontline employees getting sick, even with mild infections, was enough reason to offer them booster shots.