DEAR MAYO CLINIC: Both of my parents have been vaccinated for COVID-19. The doctor told my mom she is eligible for a third dose of a COVID-19 vaccine, but my dad is not eligible for a booster now. Can you explain who needs an additional vaccine now, and the differences between a booster and third dose.
ANSWER: It's good to know that your parents are among the millions of people who have been vaccinated for COVID-19. A primary vaccine, such as the one your parents have received, is intended to build an initial immune response.
Although viruses constantly change through mutation, and new variants can occur, such as with the COVID-19 delta variant, vaccination is an important step in the fight against the virus. Being vaccinated for COVID-19 can protect you from becoming infected and possibly infecting those around you, but it also can lessen the severity of illness if you are infected.
The Centers for Disease Control and Prevention (CDC) reported that people 65 and older who have been vaccinated have a significantly reduced risk of hospitalization if they become infected with COVID-19.
As far as the booster shot and the third dose of a COVID-19 vaccine, the main difference is really the purpose of the shot and the status of the person receiving the shot.
The primary vaccination series is intended to build an initial immune response. However, some people need more coverage.
Think of it like painting a house. You sometimes can get good coverage with one coat of paint, but occasionally you might need two or even three coats of paint to get that good coverage. It's the same way with vaccines.
In the case of COVID-19 messenger RNA vaccines, like Moderna and Pfizer, some people might have a great response to one dose. But most people need two doses of these vaccines to get a great response.
We also know that people who are immunocompromised — those whose immune systems aren't working well — often need additional vaccine doses to improve vaccination effectiveness.
In early August, it was recommended that moderately or severely immunocompromised people 12 and older who received the Moderna or Pfizer COVID-19 vaccines receive a third dose at least 28 days after completing the initial series. Immunosuppressed or immunocompromised refers to those who have medical conditions that compromise the immune system, or patients who require treatment with immunosuppressant medications, such as chemotherapy. People with a weakened immune system have a more difficult time fighting off infection than a healthy person.