evidence: A second booster dose of either the Pfizer-BioNTech or Moderna COVID-19 mRNA vaccines are recommended for people over 50. A second booster is also recommended for certain people 12 years or older who are moderately or severely immunocompromised. [...] For people over 50 and with compromised immune systems, a second booster dose should be administered four months after receiving an initial booster dose. And adults who got the Johnson & Johnson vaccine for their primary series and booster dose at least four months ago, should get a second booster dose of either Pfizer-BioNTech or Moderna mRNA COVID-19 vaccines. [...] An additional dose is often administered to people with moderately to severely compromised immune systems to improve their response to their initial vaccine series. A COVID-19 booster dose is given when a person has completed their primary vaccine series and protection against SARS-CoV-2 has decreased over time.