The U.S. Food and Drug Administration (FDA) expanded its emergency use authorization on Wednesday to include new “bivalent” COVID-19 booster shots made by Pfizer-BioNTech and Moderna. The CDC then recommended the boosters for use on Thursday. The agency recommended Pfizer’s booster for people age 12 and older, and Moderna’s for people age 18 and older.

A bivalent vaccine protects against two diseases, or two strains of the same disease. In this case, the two strains tackled by these new booster shots are the original COVID-19 strain and an omicron strain that represents an overlap between the BA.4 and BA.5 subvariants. Including protection for the original version of COVID-19 in this booster may help defend against future variants that have more in common with the first virus strain than with omicron.

These new boosters are not being restricted to those with compromised immune systems. If you have never been vaccinated against COVID-19, you will need to get an initial round of vaccination before you can get one of these new boosters.