Vaccine makers are preparing for a next possible phase of the COVID-19 vaccine rollout: booster doses. The US Food and Drug Administration on Thursday authorized third doses of both the Pfizer/BioNTech or Moderna coronavirus vaccines for certain immunocompromised people: solid organ transplant recipients or "those who are diagnosed with conditions that are considered to have an equivalent level of immunocompromise." The US Centers for Disease Control and Prevention has signed off on the authorization and now recommends an additional dose of vaccine for these certain people. But these doses will be considered part of the initial vaccination series for people who have not mounted an adequate immune response. A booster dose is something different. It helps refresh a waning immune response, or helps the body fight an evolving pathogen. And many Americans who aren't now eligible for an additional shot are wondering when it will be their turn.