COVID-19 vaccines are remarkably effective, but their protection can wane over time. That can lead to breakthrough infections in vaccinated people. A booster — an extra dose, taken several months after your last shot — can fortify your immune response to the vaccine.
Currently, everyone over 18 is eligible for a booster. Evidence suggests boosters will be a good way to maintain peak protection, particularly for those who are older, have chronic health conditions, or are at higher risk due to their occupation or where they live.
Learn more about what boosters are, how they work, and why they’re being recommended.
Vaccines train our immune systems to prevent disease. They raise infection-fighting proteins, called antibodies, to levels that block viruses or bacteria. Often, more than one dose is needed to bring antibody levels high enough to do the job. Doctors refer to these multiple shots as a series, given on a schedule over months or years. A booster is just an added dose, beyond the first series.
Many vaccines require a multi-dose series and a refresher sooner or later. For example, childhood vaccines against tetanus and whooping cough come in a scheduled series of five shots; measles requires two; hepatitis B, three. To refresh their defenses, adults get a tetanus booster every 10 years. Boosters are a customary, smart way to help our immune system stay ready.
There are two reasons to refresh your immune system with a booster. First, the number of antibody-making factories, called B cells, declines over time. After their first exposure to either a vaccine or an actual infection, some virus-specific B cells turn into long-lived “memory” B cells that remember the threat, ready to spring into action if you’re exposed again. Boosters replenish your well of antibody-making B cells by prompting more B cells — mostly memory B cells — to make antibodies. The sooner your antibody factories dwindle — which can take months, years or decades — the sooner you need a booster.
Second, viruses can also evolve variants that sidestep the antibodies stimulated by the original vaccine. Flu is caused by a virus that changes its surface proteins to avoid detection by our immune system, and flu vaccine antibodies fade in six months. So, we have to fine-tune our defenses with a new flu shot annually. We may need to do the same against SARS-CoV-2, the virus that causes COVID-19, to keep up with new variants. Although doctors don’t yet know whether COVID-19 would, like flu, require annual shots, we do know that so far vaccines remain a strong defense against hospitalization and other major complications from the COVID-19 variants.
Yes. The delta variant, which first arose in December 2020, is more than twice as contagious as the original coronavirus that emerged in Wuhan, China. While all three vaccines authorized in the U.S. were designed to stop the original strain, they are all still effective against the delta variant. Research suggests that when a vaccinated person’s antibody level wanes, they can develop a breakthrough infection — which tends to have less severe symptoms than infection in an unvaccinated person. A preliminary study showed that a Moderna booster can generate higher levels of antibodies than the original two-dose series; Pfizer boosters appear to give similar results. Based on data from a large national study, health policymakers approved switching brands when you need a booster, but they do not favor one brand of COVID-19 vaccine over another.
Individuals with compromised immune systems, about 2.7% of the U.S. population, are especially vulnerable to severe COVID-19 infections. They include cancer patients, transplant recipients, people with untreated HIV and those treated with drugs that suppress the immune system. Tests have shown that the two-dose vaccine regimens do not provide as much protection among this population. The Centers for Disease Control and Prevention has since recommended a three-dose series for those in this group with the third dose at least 28 days after the second. To protect those with weakened immune systems, it’s important that they receive boosters and for those in close contact with them to be vaccinated.
Yes. No new safety concerns have been raised for boosters. Some people can expect to experience the same discomfort that can occur after the first or second shot, such as a sore arm, fatigue, mild fever or headache. Extremely rare, more serious reactions have occurred in the initial series. The two mRNA vaccines, Pfizer and Moderna, have been associated with very rare (and usually mild) cases of heart inflammation, mostly in males 18-30 years old. Johnson & Johnson’s vaccine is associated with an extremely rare risk of blood clotting, which can be very serious, among women aged 18-49.
The right timing for a booster shot differs from person to person. The CDC says that all those ages 18 and older who received a single-dose J&J vaccine should get a booster at least two months after receiving their first shot and may choose any of the three available vaccines. Those who received two doses of the Moderna or Pfizer vaccine the first time around can get a booster, using any of the approved vaccines, at least six months after their second dose. As of November 2021, the CDC says all those in that category over the age of 18 "may" get a booster. That recommendation rises to a "should" for adults 50 and older and for those over 18 living in long-term care settings.
For the best advice, review up-to-date COVID-19 booster information from the CDC and stay in touch with your doctor. If something nudges up your risk of COVID-19, whether it is your age, a pregnancy, the health of your immune system, an underlying health problem or a high-risk exposure, your health care provider may suggest you schedule your booster sooner rather than later.
Yes. In October 2021 the CDC agreed it was acceptable to “mix and match,” taking a booster using a different vaccine brand than your first shots. Policymakers have not made any recommendations whether to switch one brand over another. Mostly, it is a matter of convenience, so people can be comfortable receiving whichever booster is available. For those concerned about rare side effects, the same low risks associated with the initial shots apply.
They’ll help, but they’re not the most important factor. Boosters can reduce the spread of COVID-19 in highly vaccinated countries and, importantly, provide people with more confidence they are fully protected. Most research suggests that the quickest way to reduce deaths and end the pandemic is to get as much of the world vaccinated as soon as possible. Providing vaccine protection to the unvaccinated will have a far greater impact on the continued spread of COVID-19 than maintaining the highest levels of protection among those who have had their first shots. Track current global vaccination rates at the New York Times COVID-19 dashboard.
We don’t know yet. Many researchers expect COVID-19 to become a fixture of our lives, much like the seasonal flu. The virus has surprised scientists with how quickly it’s mutated into new variants like delta. Although COVID-19 is changing rapidly, similar to influenza — a respiratory virus that requires seasonal vaccines — no one knows yet how COVID-19 vaccines will stand up over time. All of the COVID-19 boosters currently available are based on the original virus from late 2019, so our vaccines may need to adapt as the virus changes. Scientists are designing future boosters that target variants and organizing studies to better understand long term immunity.