Some Americans may be able to get two additional doses of messenger RNA-based COVID-19 vaccines, with the possibility of the primary series being expanded from two to three doses, Food and Drug Administration (FDA) scientists said Thursday. Moderna has sought authorization for a third dose, or a booster, for its COVID-19 vaccine and the FDA was discussing the emergency use application with its vaccine advisory panel. The FDA in August already cleared a Moderna booster, but only for certain people with weak immune systems. Dr. Michael Kurilla, a panel member who works for the National Institutes of Health, wondered how a new emergency use authorization would interact with the one for immunocompromised, especially considering the new application is for a 50-microgram dose given six months after the primary series and the one already in place is for a 100 microgram dose given one month after the initial series. In response, …