The CDC recommends that people who are moderate to severely immunocompromised receive an additional dose of an mRNA COVID-19 vaccine (Pfizer-BioNTech or Moderna) at least 28 days after completion of the initial mRNA COVID-19 vaccine series.The third dose is the same dose as the 1 st /2 nd vaccinations.. Note - This is not a booster but a third shot in the initial series..