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Doctors & Advice, Family Health

Can You Get a COVID-19 Booster? What We Know So Far

BY CARRIE MACMILLAN November 19, 2021

Booster eligibility opened up to everyone 18 and older.

[Originally published: July 27, 2021. Updated: Nov. 19, 2021]

Even though it seems like only yesterday people were calculating the date they could feel fully protected by their vaccination, now boosters are available for everyone 18 and older. 

In recent months, the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) have been busy making decisions about which groups of people are eligible for boosters, and if it’s acceptable to “mix and match” vaccines from different manufacturers for a booster dose. 

On Friday, they made their decision: All adults 18 or older are eligible for a booster shot six months after completing their primary vaccination series if they started with Pfizer-BioNTech or Moderna—or two months after getting the J&J single-shot vaccine. The approved mix-and-match policy means that any of the three COVID-19 vaccines available in the U.S. can be taken as a booster shot, regardless of which vaccine a person had for their primary vaccination. 

In a media statement, the FDA said the expansion was based on clinical trial and real-world data and to eliminate confusion about who is eligible. Furthermore, the FDA said it has been examining information about the risk of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the outer lining of the heart) following vaccination, and that it determined the benefits of a booster outweigh the risk of either condition. 

The booster conversation has gained urgency as the highly contagious Delta variant continues to infect unvaccinated individuals, and health officials around the country report breakthrough cases in fully vaccinated individuals which, though they tend to be asymptomatic or mild, are of concern.

People get confused—or they think something is wrong—when guidance changes with COVID-19, but we have to remember that we are learning about this as we go. Albert Shaw, MD, PhD, Yale Medicine infectious diseases expert

COVID-19 booster shots are not a new idea. Since the vaccines were first introduced last December, scientists have acknowledged that boosters may someday be needed

“The main question is how long the immunologic protection against SARS-CoV-2, which causes COVID-19, lasts,” says Albert Shaw, MD, PhD, a Yale Medicine infectious disease specialist. “And since we are learning about COVID-19 in real time, this is hard to know definitively.” 

For now, Dr. Shaw emphasizes that the most important thing any of us can do is get vaccinated. And if you are already vaccinated, know that the situation is being closely monitored by the scientific and public health communities. 

The recommendation of boosters doesn’t represent a failure of the existing vaccines, Dr. Shaw notes. “People get confused—or they think something is wrong—when guidance changes with COVID-19, but we have to remember that we are learning about this as we go,” he says.

We compiled a list of booster-related questions to ask Dr. Shaw. His answers are below. 

What is a booster?

“The simplest answer is that it’s just another dose of a vaccine you received,” Dr. Shaw explains. “The concept is to prolong protective immunity, particularly if there is evidence that protection is waning after a period of time.”

Most children receive routine vaccinations, including boosters, for illnesses such as chickenpox, tetanus, diphtheria, mumps, measles, and rubella—to name a few. “These vaccine series, as we call them, are recommended because you need the extra doses to get longer lasting protective immunity,” Dr. Shaw says.

There is also a technical distinction between the terms “third dose” and “booster.”  

Doctors use the term third dose when referring to people with compromised immune systems who may not have gotten the level of protection they need from the first two doses. The third dose provides that level of immunity.

A booster shot is recommended due to concern that the effectiveness of the vaccine decreases over time and may not protect against a new strain, such as Delta. A booster may be given to older people or those with chronic medical conditions or other risk factors.

Why do we need a booster for COVID-19?

While a booster sometimes is an exact replica of the initial vaccine, it can also be tweaked. With COVID-19, this is key because the vaccine could then be tailored to target particular variants of the virus.  

“The current vaccines are still effective against the variants we are now seeing, particularly for protecting against serious illness that would require hospitalization or cause death. But if the virus evolves further and there is a worse variant, the vaccine could be modified,” Dr. Shaw says. 

One of the great things about the mRNA technology, which the Pfizer and Moderna vaccines use, Dr. Shaw notes, is that it’s easy to change them up to match variants, and they can be quickly produced at scale. “This is different from the manufacturing process for the most commonly used flu vaccines, which is a much slower process because influenza virus strains need to be grown in chicken eggs, from which a particular viral protein is purified and formulated into a vaccine,” he says. 

The J&J vaccine uses a specific virus called adenovirus 26, Dr. Shaw adds. “It is engineered to be unable to reproduce itself to introduce a version of the SARS-CoV-2 spike protein—the target of the mRNA vaccines—into cells to generate a protective immune response,” he says. 

How do we know if we need a booster?

It is normal for virus-fighting antibodies—such as those that are stimulated by a COVID-19 vaccine—to wane over time. Monitoring antibody levels in the blood is one way to measure vaccine efficacy and research has found that protection remains high for six months after the second shot of a Pfizer or Moderna vaccine. However, some studies have shown a decline in antibody levels among vaccinated individuals beyond that point.

“You can certainly look at antibody levels, and that does offer some indication of how much protection lasts. But even if they have waned, that doesn’t necessarily mean the body’s capacity to respond to exposure is gone,” Dr. Shaw says. “One of the most amazing parts of the immune system is immunologic memory of past infections or vaccines. If you are re-exposed to something [via a booster shot, that follows the original exposure by vaccination], the memory response is even more vigorous than the original. This memory response includes antibody responses, but also includes an additional arm of the immune system controlled by a different group of white blood cells called T cells or T lymphocytes.”  

T cell immune responses are especially important for viral infections like those with SARS-CoV-2, but are more difficult to study than antibody responses outside of a research laboratory setting, adds Dr. Shaw. “However, T cell responses may prove to be just as important as antibody responses in protecting against infection or against serious disease requiring hospitalization,” he says. 

Can you 'mix and match' vaccines in a booster?

So-called “mixing and matching” of vaccines (a first dose of Pfizer, followed by a second dose of Moderna, for example, to complete the mRNA two-dose series) has been used in Europe and other places, particularly when there were supply issues. 

And there have been studies suggesting this approach—with one dose of AstraZeneca’s vaccine (which is not available in the U.S.) and one dose of Pfizer’s vaccine—may even offer more vigorous protection. But here in the U.S., the current public health recommendations are that people should stick with one type of mRNA vaccine for both doses. 

But what about for boosters—if they are recommended, should you stick with your original kind? 

The FDA and CDC recently supported a “mix-and-match” approach that allows people to choose a different vaccine for their booster than the one they started with. 

Can a booster cause more—or worse—side effects?

If you were among the unlucky recipients who felt really ill or had any of the rare but largely harmless reactions to your initial COVID-19 vaccination, you may be leery of the idea of a third dose, in case it causes a similar or worse reaction. 

But according to the FDA, the most commonly reported side effects by individuals who received a booster dose included pain, redness, and swelling at the injection site; fatigue; headache; chills; and muscle or joint pain. Swollen lymph nodes in the underarm were observed more frequently following the booster dose than after the second dose.

Are those who are immunocompromised eligible for a booster or a third dose?

This summer, the FDA amended the emergency use authorizations (EUA) for Pfizer and Moderna to allow a third dose of their COVID-19 vaccines for certain immunocompromised people. (An EUA is given to products during public health emergencies, but is not a traditional approval. Although Pfizer has been given full authorization for its vaccine for ages 16 and up, boosters from all three vaccine manufacturers are still under an EUA.) 

The list for immunocompromised people who are eligible for a third dose include solid organ transplant recipients and those with conditions that give them an equally reduced ability to fight infections and other diseases. A third dose of the Pfizer or Moderna vaccines may increase protection in this vulnerable population, according to data the FDA evaluated.  

Should we first be vaccinating the world?

There is also a moral element to the question of booster shots, with some public health officials saying the focus should be on vaccinating more of the world’s population first before giving supplementary doses to those who already have some protection.

Dr. Shaw says he understands this sentiment. “On this planet, we are all interdependent and we can see that many of these variants originated outside the U.S. That certainly speaks to the need to vaccinate the world, especially to drive down infection rates that support the emergence of new variants," he says. "Ultimately, this is what needs to be done to end the pandemic.”

[Visit Yale Medicine's Vaccine Content Center for more stories on COVID-19 vaccines.]