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A Booster Dose or a Third Dose of the COVID Vaccine: Is There a Difference?


The Centers for Disease Control and Prevention (CDC) recently recommended an additional, booster dose of the COVID-19 vaccine for all people over the age of 18 after their initial two-dose vaccine series. This is different from the third dose that was recommended to those who are immunocompromised. Oladipo Alao, MD, MPH, FACP, a Board Certified Infectious Disease and Internal Medicine Physician at Crystal Run Healthcare, provided some clarification between the two doses.

What is a third dose?

“A third dose is an additional COVID-19 vaccine shot that is administered four weeks after the second dose,” explained Dr. Alao, “This has been recommended to moderately to severly immunocompromised people as studies suggest that the typical vaccination dose is not as robust for those that are immunocompromised.”

It is recommended that those receiving a third dose of the Moderna or Pfizer mRNA vaccine as part of their primary series receive the same vaccine for each dose. Those who received the Johnson & Johnson COVID-19 vaccine will also need an additional dose.

Who should get a third dose?

According to the CDC, those who have received an organ transplant, those who are being actively treated for cancer, those on drugs that suppress the immune system, and those with untreated or advanced HIV are included in the list of people who need a third dose of the Pfizer and Moderna vaccines. Studies have also found that these immunocompromised individuals are more vulnerable to COVID-19 and its variations, account for a significantly higher than expected number of hospital admissions with ‘breakthrough’ infections, and are more likely to be infectious and transmit COVID-19 as a result. 

“Talk to your provider to see if you are eligible for a third dose, based on your condition,” recommends Dr. Alao, “While a third dose of vaccine is supposed to provide better protection from COVID-19 compared with two doses for certain individuals, it is not known if it will provide the same level of immunity as is seen in an average healthy person. Vulnerable individuals will still need to continue to take precautions such as wearing a mask, especially inside, and avoiding large crowds.”

What is a booster dose?

A booster dose is also an additional dose; however, it is given to healthy individuals to counteract the natural decrease in antibodies. According to the CDC, protection against COVID begins to decrease over time after six to eight months from the second vaccination shot. This decreased immunity, along with the dominance of the Delta and Omicron variant, will lead to reduced protection against the COVID-19 virus and an increase in breakthrough cases. A booster dose is recommended six months after the second dose to maximize the durability of your immune system’s response if you received the Pfizer or Moderna (mRNA) vaccine series. If you received the Johnson & Johnson series it is recommended you receive a booster dose at least two months after completing your primary vaccination. CDC recently changed the age recommendation for those who should receive a booster dose to individuals 18+.

Do you need a third dose or booster dose?

To summarize, a ‘third’ dose is given 28 days after the second dose, to individuals who are vulnerable and may have reduced immune response to two doses, so they can achieve improved levels of immune response. In contrast, a ‘booster’ dose is an additional dose, given two to six months after a complete series, depending on what vaccination was initially received, to individuals without conditions who have an initial immune response but may have reduced levels of immunity over time, to maintain high levels of immunity. Your Primary Care physician or the Infectious disease specialists at Crystal Run Healthcare can help you determine if getting a third dose or a booster dose is right for you. To schedule an appointment with a Crystal Run provider, visit

For more information about the COVID-19 vaccine or to schedule a vaccine appointment at one of Crystal Run Healthcare’s locations in Orange, Sullivan, and Rockland counties, visit Oladipo Alao, MD, MPH, FACP, is a Board Certified Infectious Disease and Internal Medicine Provider at Crystal Run Healthcare. Dr. Alao treats people of all ages and has clinical interests in HIV, travel medicine, infectious diseases, and internal medicine. Dr. Alao is seeing patients at Crystal Run’s Monroe and West Nyack offices.