Americans aged 65 and older who received the two-dose Pfizer-BioNTech vaccine should get a third booster dose of that vaccine if it has been at least 6 months since completing their vaccination series.That was the recommendation the Food and Drug Administration (FDA) made late yesterday, and it was echoed today during a meeting of the Centers for Disease Control and Prevention's (CDC's) Advisory Committee on Immunization Practices (ACIP).In addition to those 65 and older, the FDA authorized booster doses of the Pfizer vaccine for people 18 through 64 who are at high risk of severe COVID-19 or whose institutional or occupational exposure puts them at high risk of severe COVID-19.