The FDA has approved an additional COVID booster from Pfizer and Moderna for those who are 50 years old and older.

The decision opens up the fourth dose for anyone who is at least four months removed from their previous booster. Until now the fourth shot was only available to those 12 and older with severely weakened immune systems. The agency also says that people in that circumstance can now receive an additional booster, their fifth shot.

About two thirds of Americans are fully vaccinated, and half of those who are eligible for their first booster have not received it.