Millions of Americans are now eligible for COVID-19 boosters. However, it can be difficult to determine who is eligible and when. The added challenge is the fact that people can choose to get an extra dose of a different vaccine this time.

When you are eligible for the vaccine depends on many factors. These include when you last received it and what kind of vaccine you had. Just like your initial shots, boosters can be obtained at no cost at clinics, pharmacies, and doctors' offices.

These are some facts to be aware of:

WHY DO BOOSTERS ARE NEEDED?

Even though they are fully vaccinated, they are strongly protected from being hospitalized or dying due to COVID-19. However, immunity to infection can decrease over time and the extra-contagious Delta variant is rapidly spreading. U.S. health officials want to increase protection for at-risk individuals who were vaccinated months back, but they stress that it is still important to get the unvaccinated their first shot.

BOOSTERS AVAILABLE TO ALL THREE VACCINES AUTHORIZED IN The U.S.

Yes, Pfizer boosters started last month. This week, the government approved additional doses of the Moderna or Johnson & Johnson vaccines. Officials say that the next few months will see more than 120 million Americans eligible for boosters. That's about 2/3 of all adults who have been vaccinated. The eligibility criteria and the time frame for a booster vary depending on which vaccine was given first.

CAN I GET A BOOSTER NOW?

You are eligible if you have had Moderna or Pfizer shots before. For example, health care workers are eligible because they are frequently exposed to the virus and are unable to travel with the mildest infection.

WHAT IF THE J&J SHOT IS MADE?

Anybody who has had a J&J shot within the last two months is eligible regardless of their age or other factors.

WHY DO DIFFERENT RECOMMENDATIONS APPEAR FOR DIFFERENT VACCINES?

One shot of the J&J vaccine has a lower effectiveness than two doses each of Moderna and Pfizer, so health authorities determined that J&J recipients should have the same level of protection. J&J tested more people with a two month booster than with one at six months. Moderna and Pfizer recipients have not been shown to require another dose. However, immunity to infection did seem to decrease in some individuals after six months.

WHAT IF I DO NOT WANT TO WAIT SIX MOTHS?

Experts agree that a booster administered too soon may reduce its effectiveness. Because the immune system builds layers over time, timing is important. This will increase the likelihood that a later dose will offer greater protection.