What to Know About Getting a COVID-19 Booster Shot

An infectious diseases expert explains the safety, side effects, and mixing and matching of COVID-19 vaccines.

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 everyone age 18 and older.

After initially recommending the booster for only higher-risk populations, health officials have streamlined the guidance and expanded the eligibility. Any adult can get a booster if they received:

  • Either the Pfizer/BioNTech or the Moderna mRNA vaccine and completed their primary series at least six months ago
  • The Johnson & Johnson single-dose shot at least two months ago

The CDC cited data from small clinical trials that showed that while vaccines continue to protect against severe disease and hospitalization, their effectiveness at preventing COVID-19 infection appears to wane over time. Furthermore, a clinical trial of the Pfizer/BioNTech’s COVID-19 vaccine booster showed that it is effective in protecting against COVID-19, indicating that a booster dose may result in increased protection compared to just having the initial vaccination series.

Dr. Marcus Pereira

Dr. Marcus Pereira

“The CDC and FDA were waiting on sufficient data to show that booster shots are safe and recommended for the general adult 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.”

Health Matters spoke with Dr. Pereira about the latest guidance and data on booster shots.

What have the FDA and CDC decided about COVID-19 booster shots and why?
After the White House made an initial recommendation for booster shots two months ago, scientists from the FDA and CDC have closely monitored data and confirmed that vaccine protection may decrease over time. For example, studies in the United States have found that among essential workers, vaccine effectiveness against infection waned in July, though severe disease remained rare.

For this reason and because of the rise of the more contagious Delta variant, a booster shot is now recommended for anyone over the age of 18 who finished their mRNA series at least six months ago or received the Johnson & Johnson vaccine at least two months ago.

By changing the eligibility criteria, the guidance is now less confusing. They’re telling the general public, absolutely get a booster. And the time to do it is now, before the holidays and the new year.

Are booster shots safe?
Yes, booster shots are proven to be safe. Pfizer released a study of 10,000 participants in which half of them received a booster dose and half a placebo. In terms of safety, they found no new adverse events, meaning it was consistent with what has been seen in previous studies. For those concerned about myocarditis or pericarditis, inflammation of the heart muscle or the outer lining of the heart, no cases of either were observed. In fact, the placebo group had more serious adverse events than the booster group.

In addition, 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 the boosters’ safety. The benefit outweighs any risks of receiving either the Pfizer or the Moderna booster dose.

Are booster shots effective?
The booster shots are not only safe, they are also shown to be very effective in boosting one’s immunity to COVID-19. In the Pfizer study, they found that the relative vaccine efficacy after the booster was more than 95%, which restores the original vaccine efficacy before immunity waned.

Moderna is still accumulating final data, but since Moderna has been shown to produce a good amount of neutralizing antibodies and total antibodies, we expect the efficacy to be on par with the Pfizer results.

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.

“By changing the eligibility criteria … they’re telling the general public, absolutely get a booster. And the time to do it is now.”

— Dr. Marcus Pereira

If you had COVID-19 and were fully vaccinated, do you still need to get a booster?
Pfizer did study those who had prior COVID infection, and the data suggests that a booster dose is protective for that population and certainly not harmful.

Are there any side effects to the booster?
Pain at the injection site and headaches were commonly reported side effects, but the booster dose has been shown to have a lower likelihood of side effects than the second dose.

The CDC has been monitoring injection site reactions and systemic reactions (fatigue, fever, body aches) as well as health impacts, such as the ability to work or perform daily activities. In terms of these factors, the reactions from the booster appear to be less disruptive than the reactions experienced from the second 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 approved vaccines. 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.

The National Institutes of Health released a study on mixing and matching that looked at antibody data. It found that the mRNA vaccines resulted in higher antibody titers in the first 28 days after the boost. If you were to really split hairs, getting a Moderna booster looks to produce slightly better antibodies of the three, but not by much compared to Pfizer — it’s almost tied. But if I got the Johnson & Johnson, I’d choose a Moderna or Pfizer booster.

Also, remember this data is based on antibodies, so what it means in the real world is hard to know. It is up to each individual to determine which type of vaccine to get as a booster shot, so I’d recommend speaking to your doctor for more specific guidance.

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 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 and when they encounter that same virus or bacteria again.

B-cells
Function: Produce antibodies.
Strengths: Create memory cells and are primed to respond if and 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.