COVID-19 booster shots have now been approved for many adults, whether you got the Pfizer, Moderna, or Johnson & Johnson vaccine.

Federal health officials have declared that adults over age 18 who received Johnson & Johnson’s vaccine should get a booster after two months because the initial vaccine is less effective than the two-dose Pfizer and Moderna vaccines. Johnson & Johnson’s vaccine is 71% effective against hospitalization, compared to 93% for Moderna and 88% for Pfizer, according to CDC data.

“I think this, frankly, was always a two-dose vaccine,” Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia and a member ofan FDA advisory panel, said of the J&J vaccine during a recent meeting of the group. “It would be hard to recommend this as a single-dose vaccine at this point.”

Here’s what to know about COVID-19 vaccine boosters:

Why is a COVID-19 booster necessary?

Vaccines work by spurring the development of antibodies that protect against a virus. A booster shot is an extra dose to “boost” immunity by prompting the body to develop more antibodies.

The two-dose Pfizer and Moderna vaccines, and the single-shot Johnson & Johnson vaccine, are highly effective in protecting people against the original COVID-19 strain. However, boosters are being recommended because new studies show that protection may decrease over time.

Boosters would also offer more protection against the highly transmissible delta strain. The original vaccines have been effective in preventing severe illness and hospitalization due to the delta strain, though some fully vaccinated people have developed “breakthrough” cases with minor symptoms.

Who will be eligible for a Moderna booster?

The FDA has approved the same people who are eligible for a Pfizer booster to get a Moderna booster after six months:

  • People 65 and older

  • Adults with compromised immune systems

  • People with greater risk of workplace exposure

See below for more details on who falls into these groups.

Boosters are not currently recommended for children.

Who will be eligible for a Johnson & Johnson booster?

The FDA approved booster shots for all adults age 18 and older who received a Johnson & Johnson COVID-19 vaccine at least two months ago.

According to data Johnson & Johnson presented to the FDA advisory panel, the vaccine’s effectiveness increased from 74% to 94% when a second dose was given two months later.

Who is eligible for a Pfizer or Moderna booster?

Pfizer and Moderna booster shots are currently available for certain groups who completed their initial two-dose regimen at least six months ago:

  • Adults 65 and older

  • Adults age 50 to 65 with underlying medical conditions

  • Long-term care residents

  • Adults age 18 to 49 with underlying medical conditions, depending on their individual risks of exposure

  • Adults age 18 and older with high risk of workplace exposure, such as first responders; medical professionals; teachers; corrections officers; grocery store workers; and people in public transit, agriculture, manufacturing, and the U.S. Postal Service

Should I get a booster of my original vaccine, or is it OK to go with a different company’s booster?

The agency authorized those eligible for a booster to receive an additional shot of any of the three approved vaccines, a practice known as “mix and match.” In other words, people who got the Moderna vaccine could seek a Pfizer booster, or people who received the Johnson & Johnson vaccine could get a Moderna booster.

» READ MORE: An illustrated guide to how the COVID-19 vaccines work

Where can I get a booster shot?

The easiest way to find an appointment for a booster shot may be to contact the pharmacy or doctor’s office where you got the initial vaccine.

Find a vaccine appointment with the CDC’s online search tool at vaccines.gov, or by texting your zip code to 438829, or calling 1-800-232-0233.