Although it wasn’t too long ago that we last covered the announcement of COVID-19 booster shots, a lot has changed since then—in particular, who can get the additional dose of the vaccine, and the parameters in which they can do so. Additionally, now that booster shots are underway, more research can be done on the effectiveness of additional doses of the COVID-19 vaccine.  

“As more real-world data is collected on the effectiveness of the COVID-19, alongside the emergence of more potent SARS-CoV-2 variants (such as the Delta variant), it follows that booster doses are recommended for some groups, particularly for those who got the Johnson & Johnson vaccine—which has shown to be slightly less effective compared with the Pfizer and Modern vaccines (both mRNA vaccines),” says STChealth’s Chief Epidemiologist and Chief Scientific Officer, Dr. Kyle Freese, PhD, MPH.  


As a result of the Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) recommendation for a COVID-19 vaccine booster shot for certain qualifying groups—as well as the U.S. Food and Drug Administration’s (FDA) Emergency Use Authorization (EUA) for the use of a single COVID-19 booster dose—an increasing number of Americans are eligible to receive the additional dose. 

Among the groups recommended to get a COVID-19 booster vaccine are:  

  • Individuals 65 years or older 
  • Individuals ages 50-64 years old with underlying medical conditions 
  • Individuals 18 years or older who received the Johnson & Johnson single-dose COVID-19 vaccine 
  • Individuals 18 years or older who are immunocompromised or are otherwise at high risk of contracting severe COVID-19. For a full list of CDC recognized underlying health conditions, click here.  
  • Individuals 18 years or older who have frequent institutional or occupational exposure to SARS-CoV2 (the virus that causes COVID-19) 
  • Individuals 18 years or older who live in long-term care settings 

According to an announcement published by the FDA, “To support the authorization for emergency use of a single booster dose of the Moderna COVID-19 Vaccine, the FDA analyzed immune response data from 149 participants 18 years of age and older from the original clinical studies who received a booster dose at least six months after their second dose and compared it to the immune responses of 1,055 study participants after completing their two-dose series. The antibody response of the 149 participants against SARS-CoV-2 virus 29 days after a booster dose of the vaccine demonstrated a booster response.  

The FDA also evaluated an additional analysis from Moderna comparing the rates of COVID-19 accrued during the Delta variant surge during July and August 2021, which suggest that there is a waning of vaccine effectiveness over time.” 


Individuals eligible to receive the COVID-19 booster shot can now choose from either the Pfizer or Moderna mRNA COVID-19 vaccines for their additional shot—even if said vaccine manufacturer is different from that of their initial COVID-19 vaccine dose(s). Thanks to this update, booster shots are now more widely available, and those who initially received the one-shot Johnson & Johnson vaccine can choose from either the Pfizer or Moderna vaccines for their booster shots, which have a higher vaccine effectiveness over time compared to the Johnson & Johnson COVID-19 vaccine.  

Those who are eligible to get an COVID-19 booster shot should wait at least six months after their second shot before doing so if they previously got the Pfizer or Moderna vaccine, or two months if they previously got the single dose Johnson & Johnson vaccine.   


According to the CDC, reactions reported COVID-19 booster shots have been similar to that of both the Pfizer and Moderna initial vaccine series. The most commonly reported side effects have been fever, headache, fatigue and pain at the injected sites. Overall, the majority of reported side effects have been mild to moderate.   


“Overall, building population-wide immunity is the goal,” says Dr. Freese. “Though encouraging individuals who have no immunity to be vaccinated would have the highest rates of return, maximizing protection—particularly among vulnerable groups such as those who are elderly or immunocompromised—is also important.”