What to Know About COVID-19 Booster Shots: Safety, Side Effects, and Who Should Get One

An infectious disease expert explains the latest COVID-19 booster shot recommendations from the CDC and FDA.

The Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) have now authorized and recommended COVID-19 booster shots for a larger segment of the population.

After initially providing guidance for only those who received the Pfizer vaccine, health officials are now recommending boosters for people who have completed their second dose of either the Pfizer or Moderna vaccines at least six months ago and who meet one of the following criteria:

  • 65 years and older
  • Age 18+ who live in long-term care settings
  • Age 18+ who have underlying medical conditions
  • Age 18+ who work or live in high-risk settings

The CDC also recommends that people who are 18 or older and got the Johnson & Johnson vaccine receive a booster shot two months after their initial dose.

“The CDC and FDA were waiting on sufficient data to show that booster shots are safe and recommended for this population, which they now have,” says Dr. Marcus R. Pereira, medical director of the Transplant Infectious Diseases Program at NewYork-Presbyterian/Columbia University Irving Medical Center and assistant clinical professor of medicine in the Division of Infectious Diseases at Columbia University Vagelos College of Physicians and Surgeons. “These actions were based on new data showing an increase in the immune response after a booster shot.”

Dr. Marcus Pereira

Dr. Marcus Pereira

Health Matters spoke with Dr. Pereira about the latest on who can receive a booster shot.

What has the FDA and CDC decided about COVID-19 booster shots, and why?
After the White House made the recommendation for booster shots, scientists from the FDA and CDC reviewed the data and confirmed that vaccine protection may decrease over time, putting the most vulnerable at risk for more severe infection.

For this reason — and because of the rise of the more contagious Delta variant — a booster shot is recommended for people who are at an increased risk.

For the general population who have healthy immune systems, antibodies, and what are known as memory B and T cells — cells that remember viruses and can quickly activate antibody formation when triggered — the research indicates that they are still very much protected by the vaccine they received. Booster shots are not recommended for the general population at this time.

It should be noted that the Moderna booster is only half the original dose. This is because the initial two doses of the Moderna vaccine were much higher comparatively speaking than the Pfizer, so individuals need only half a dose for boosting.

The CDC recommended some people “should” get the booster shot, while others “may” get it. Can you explain the breakdown
Yes, it’s interesting how the CDC worded their recommendation, but it boils down to this: The CDC recommends that people who are at least six months past getting their second dose of the Pfizer or Moderna vaccines should receive a booster shot if they are age 65 or older, or 18 years and older in long-term care settings like nursing homes, or 50 to 64 years old with underlying medical conditions.

People who are at least six months past getting their second dose of the Pfizer or Moderna vaccine and 18 to 49 years old with underlying medical conditions and people 18 to 64 who are at increased risk of COVID-19 exposure because of their occupation or where they work—for instance, healthcare or frontline workers—may receive a booster shot.

The CDC cites a long list of health conditions that place someone in a position to get a booster dose. Those include:

  • Cancer
  • Chronic kidney disease
  • Chronic lung diseases like COPD, asthma, and cystic fibrosis
  • Neurological conditions like dementia and Alzheimer’s
  • Diabetes
  • Down syndrome
  • Liver disease
  • Obesity
  • Heart conditions like coronary artery disease and hypertension
  • Pregnancy

These are the categories — which include a substantial number of people — that the CDC authorizes to receive a booster. The complete list can be found here. Talk to your doctor about whether a booster shot would benefit you.

What about the Johnson & Johnson vaccine?
For the nearly 15 million people who received the J&J vaccine, the CDC recommends getting a booster shot if you are 18 and older and two months have past since you received the initial dose.

Can you mix and match vaccines?
The FDA and CDC now allow for a mix-and-match approach for booster doses. That means that those who are eligible to receive a booster shot have the option of getting any of the vaccines available for boosting. Data show that this is safe and effective and, in some cases, particularly in the case of the J&J vaccine, mixing and matching elicits a stronger immune response.

It is up to each individual to determine which type of vaccine to get as a booster shot. Speak to your doctor for more specific guidance.

“Booster shots are proven to be safe and effective in boosting one’s immunity to COVID-19. The CDC found that reactions from the third dose were very similar to those after the second dose.”

— Dr. Marcus Pereira

What studies did health officials evaluate to arrive at this recommendation?
The decision was based on several studies. For example, a recent study conducted in Israel this summer found that people who received a third dose of the Pfizer vaccine showed greater protection against the coronavirus compared to those who only received two doses. Furthermore, studies in the United States have also found that among essential workers, vaccine effectiveness against infection waned in July, though severe disease remained rare.

Are there any side effects to the booster?
Fatigue and pain at the injection site were the most commonly reported side effects. Interestingly, booster doses have shown to have slightly reduced side effects, with less prevalence of chills, fever, or body aches. We’re not exactly sure why this is, but it could be because the body’s immune system is already “primed,” so the side effects are not as severe with the booster shot.

Are booster shots safe?
Yes, booster shots are proven to be safe and effective in boosting one’s immunity to COVID-19. The CDC found that reactions from the third dose were very similar to those after the second dose, stating that there are “no unexpected patterns of adverse reactions” and they will continue to monitor its safety.

The Immune System’s Vital Components

The immune system is a highly complex system that helps protect the body from disease. There are three essential components for a healthy immune response:

Antibodies
Function: Identify pathogens/viruses when they first enter the body.
Strengths: Send a cascade of alarm signals down to the immune system and help to neutralize the virus or bacteria.

T-Cells
Function: Find and destroy cells that are infected with a virus or bacteria.
Strengths: Create memory cells and are primed to respond if/when they encounter that same virus or bacteria again.

B-Cells
Function: Produce antibodies.
Strengths: Create memory cells and are primed to respond if/when they encounter that same virus or bacteria again.

Memory B-Cells and T-Cells
Because antibodies naturally decay over time, memory B-cells and T-cells allow a person’s immune system to remember a specific virus they were once exposed to. These memory cells are constantly circulating in the body, and they are primed to respond by creating new antibodies if they are reexposed to a virus or bacteria. These new antibodies will then send alarm signals to the immune system to neutralize and kill the virus.

Additional Resources

  • Learn more about the COVID-19 vaccine.

    Schedule to get a booster shot at NewYork-Presbyterian.

Marcus R. Pereira, M.D., MPH, is medical director of the Transplant Infectious Diseases Program at NewYork-Presbyterian/Columbia University Irving Medical Center and an assistant clinical professor of medicine in the Division of Infectious Diseases at Columbia University Vagelos College of Physicians and Surgeons. Dr. Pereira is focused on infectious complications in patients with solid organ and bone marrow transplants as well as hematological malignancies, and provides care in both inpatient and outpatient settings. As medical director of the Transplant Infectious Diseases Program, Dr. Pereira oversees the development of infection prophylaxis and treatment protocols for immunocompromised patients. His areas of interest include multi-drug-resistant infections, including bacterial and fungal organisms in transplant patients as well as resistant cytomegalovirus infections. He is an associate editor of the American Journal of Transplantation and has led several studies this past year on the impact of COVID-19 in solid organ transplant recipients.