Some people may not be able to mount an adequate immune response with the traditional primary series. It could be they are immunocompromised from a medical condition or from medications they take. In those cases, they may be eligible for an extra dose in their primary vaccine series.
A third dose of the Pfizer or Moderna vaccine can be administered within 28 days for the two-dose series and is covered by Medicare. For anyone who is immunocompromised, this is considered part of the primary series and is not a booster dose.
The CDC currently recommends that people who are immunocompromised and who received the Johnson & Johnson vaccine as their primary vaccine get a second vaccine dose at least four weeks after their first shot to complete their primary series. A J&J vaccine can be given as the second shot but a dose of Pfizer or Moderna is preferred. Again, this will be considered part of the primary series and is covered free of charge.
Additional doses in the primary series are not recommended for the Novavax vaccine at this time.