Some immunocompromised Americans can get four doses of COVID-19 vaccine

While most Americans who initially received two shots of the Pfizer or Moderna COVID-19 vaccines will be able to get a single booster shot when they are eligible, federal health guidelines will also allow a few million Americans with compromised immune systems to eventually get a fourth dose as early as next year. 

Back in August, the FDA and the Centers for Disease Control and Prevention authorized and recommended an "additional" third shot for some "moderately and several immunocompromised" patients who were vaccinated with two doses of Moderna or Pfizer. 

Now some of those people will be eligible to receive a fourth shot — a "booster" six months after they received their third "additional" dose.