Some people with compromised immune systems who received Moderna or Pfizer's mRNA COVID-19 vaccine may get a fourth shot, according to CDC guidance updated Oct. 25.
The guidance makes a distinction between an "additional" dose and a "booster" dose, noting that moderately and severely immunocompromised adults ages 18 and older — who may not have mounted a strong immune response after the initial series — are eligible for an additional or third mRNA dose. After the third dose they are eligible for a single booster dose, making it the fourth shot for some immunocompromised people.
Booster doses, which may be any of the COVID-19 vaccines approved for use in the U.S., should be administered at least six months after the third mRNA vaccine dose.
For most of the general population, their third shot would be considered a booster dose.
Pfizer COVID-19 vaccine recipients became eligible for a booster dose Sept. 24, and the CDC endorsed booster shots for Moderna and Johnson & Johnson recipients Oct. 21. The CDC's approval also allows mixing vaccine doses from different manufacturers, so people can choose which booster shot they want to receive.