evidence: There are a number of questions surrounding COVID-19 vaccines that remain unanswered. Some of these have to do with the biology of the virus and/or host immune system, including how long the vaccine will remain protective and will boosters be needed? Scientists are actively researching the immunopathology of SARS-CoV-2, and although data about the longevity of immunity look promising against early variants, the virus is continuing to evolve. While vaccines remain highly effective at preventing the worst outcomes of SARS-CoV-2 caused by the highly transmissible Delta variant, including severe disease, hospitalization and death, vaccinated sera show reduced neutralization activity against Delta. And we still do not know how long vaccines will remain protective.The CDC has recommended a third dose of mRNA vaccine, at least 28 days after administration of the second dose of Pfizer or Moderna COVID-19 vaccines, for people who are moderately to severely immunocompromised.