A third dose of the CoViD-19 vaccine is given to individuals with compromised immune systems because it is likely that the first two doses did not stimulate an adequate immune response. The third dose should make those individuals “fully vaccinated.”
A booster dose is a supplemental vaccination given to fully vaccinated individuals after their immune system response has weakened (typically an extended period of time – for instance, Tetanus vaccine boosters are recommended every ten years.) Booster shots against CoViD-19 are not yet recommended for fully vaccinated individuals. (In Nassau County, the first residents to be vaccinated with Moderna’s brand did not receive their second doses until the end of January; boosters will not be needed until at LEAST eight months afterward.)
However, it is recommended that those who are immunocompromised (due to an autoimmune disease or immunosuppressive therapy medications) receive a third dose of CoViD-19 mRNA vaccine 4+ weeks after their second dose in order to be fully immunized.
The vaccine given should be the same brand for all three doses.
DOH-Nassau has the Pfizer brand (approved for anyone age 12 years and older). Most area pharmacies have the Moderna brand (because it is easier to store; only approved for 18+ years of age).
There is no shortage of CoViD-19 vaccine – there is no need to mix vaccine brands for the third dose.