[Originally published: July 27, 2021. Updated: May 20, 2022.]
Note: Information in this article was accurate at the time of original publication. Because information about COVID-19 changes rapidly, we encourage you to visit the websites of the Centers for Disease Control & Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information.
Even though it seems like only yesterday people were calculating the date they could feel fully protected by their COVID-19 vaccination, boosters are increasingly being recommended for wider swaths of the population.
In fact, now a second booster is an option for many Americans. Recently, the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) authorized a second booster shot of the Pfizer-BioNTech and Moderna vaccines for everyone 50 and older and for people with certain conditions that make them immunocompromised.
Additionally, the CDC says all adults who completed a primary vaccine and booster dose of Johnson & Johnson’s vaccine should now receive a second booster from either Pfizer of Moderna. In all cases, a second booster can be administered at least four months after the first booster.
This latest guidance on boosters comes on the heels of many changing recommendations. In November 2021, the FDA and CDC said all adults 18 or older are eligible for a booster shot six months after completing their primary vaccination series if they started with Pfizer or Moderna—or two months after getting the Johnson & Johnson single-shot vaccine.
In December 2021, the FDA authorized (and CDC approved) the Pfizer booster for 16- and 17-year-olds. But for J&J recipients, the advice changed a few weeks later when the CDC recommended that vaccines other than J&J’s should be preferred, citing links between the vaccine and a rare blood clotting disorder.
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
Then, in early May this year, the FDA severely restricted use of J&J, citing the risk for the blood-clotting disorder. The FDA said J&J should be used only for those unable to receive another vaccine because it is “not acceptable or clinically appropriate” or for those who would otherwise not receive another vaccine.
Furthermore, the FDA and CDC in late May authorized and recommended a booster dose of the Pfizer vaccine for children ages 5 to 11 at least five months after their second dose. A Pfizer booster is already approved for adolescents ages 12 to 15 at least five months after their second dose. (Moderna is only authorized for ages 18 and up.)
Meanwhile, boosters became increasingly important as the highly contagious Omicron variant caused a surge in cases last winter, and now there are concerns about a BA.2, a subvariant of Omicron that swept across Europe in March.
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.”
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. 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.
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 now support a “mix-and-match” approach that allows people to choose a different vaccine for their booster than the one they started with. However, the CDC now says mRNA vaccines are preferred over J&J as a booster, though J&J can still be considered in some situations.
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 an additional 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.
The FDA 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 with mRNA vaccines, and has determined the benefits of a booster outweigh the risk of either condition.
What are the recommendations for those who are immunocompromised?
The FDA and CDC have authorized a second booster from Pfizer and Moderna for people with certain immune deficiencies. This includes solid organ transplant recipients and those with conditions that give them an equally reduced ability to fight infections and other diseases. Boosters may increase protection in this vulnerable population, according to data the FDA evaluated.
For those 12 and up, this means a three-dose primary series of an mRNA vaccine and two boosters—for a total of five shots. But for children between ages 12 and 18, boosters must be from Pfizer only, as the Moderna vaccine is not approved for that age group.
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.”
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