Some Americans can get four COVID-19 vaccine doses within less than a year, the Centers for Disease Control and Prevention (CDC) said this week. U.S. drug regulators and the CDC recently cleared booster doses of all three COVID-19 vaccines authorized for use in the country. The fourth dose would be for people who are either moderately or severely immunocompromised, or people with weakened immune systems, the CDC said in updated clinical guidance. This population is allowed to get a third dose of a Pfizer or Moderna vaccine at least 28 days after their second dose. Both vaccines are built on messenger RNA technology and have a two-dose primary series. The population can then get a booster six months or longer after their third dose, according to the CDC. “In such situations, people who are moderately and severely immunocompromised may receive a total of four COVID-19 vaccine doses,” the agency says. …