An additional primary shot of a COVID-19 vaccine is recommended for people who are vaccinated and might not have had a strong enough immune response. In contrast, a booster dose is recommended for people who are vaccinated and whose immune response weakened over time. Immune response fades naturally over time. It can also happen when the virus that causes COVID-19 changes so the immune system doesn’t recognize it as well. Research suggests that getting a booster dose can decrease your risk of infection and severe illness with COVID-19.

The CDC recommends additional primary shots and booster doses of COVID-19 vaccines in specific instances:

• Additional primary shot. For people age 5 and older, the CDC recommends a third dose of an mRNA COVID-19 vaccine for some people with weakened immune systems, such as those who have had an organ transplant. People with weakened immune systems might not develop enough immunity after vaccination with two doses of an mRNA COVID-19 vaccine or one dose of the Janssen/Johnson & Johnson COVID-19 vaccine. An additional shot using an mRNA COVID-19 vaccine might improve their protection against COVID-19.

  The CDC now recommends that children ages 5 through 11 who have moderately or severely weakened immune systems should get an additional primary shot of the Pfizer-BioNTech COVID-19 vaccine.

  The additional primary shot should be given at least four weeks after a second dose of an mRNA COVID-19 vaccine or one dose of the Janssen/Johnson & Johnson COVID-19 vaccine. The additional shot should be the same brand as the other two mRNA COVID-19 vaccine doses that were given. If the brand given isn’t known, either brand of mRNA COVID-19 vaccine can be given as a third dose.

• Booster dose. These recommendations differ by age, what vaccines you have been given and the state of your immune system. But in general, people can get the booster shot at least two months after their last shot. People who recently had a positive COVID-19 test may think about waiting three months after their symptoms started to get the booster.

  Kids ages 6 months through 5 years who got the Moderna COVID-19 vaccine can get an updated, called bivalent, Moderna COVID-19 vaccine booster. The booster is based on the original virus strain and two omicron strains.

  Kids age 5 who got the Pfizer-BioNTech COVID-19 vaccine can only get the updated Pfizer-BioNTech COVID-19 bivalent vaccine booster. Kids age 6 and older can choose between the Pfizer-BioNTech and the Moderna COVID-19 bivalent vaccine boosters.

  If you are age 18 or older, and got the Janssen COVID-19 vaccine, you can get either of the mRNA vaccine bivalent boosters at least two months after your shot. People 18 and older may also get a Novavax booster based on the original virus strain as a first booster at least six months after their last shot.

  Pregnant people can also get a COVID-19 booster dose.

  People age 5 and older who have a weakened immune system and have had all recommended doses of the Moderna, Pfizer-BioNTech or Novavax COVID-19 vaccines can get a booster dose of the updated Pfizer-BioNTech or Moderna COVID-19 bivalent vaccines.