COVID-19 Vaccine Boosters

What You Need to Know

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.

New boosters reformulated to target highly transmissible omicron variants are now available. Until September 2022, the only vaccines offered were those designed to block the original COVID-19 virus. But that virus evolved ways to evade immunity more quickly than expected, notably with the rise of omicron and its sister variants. The reformulated “bivalent” boosters refresh the original vaccine protection and contain new ingredients that target omicron subvariants BA.4 and BA.5, which caused most infections during the summer of 2022. Already been boosted with the older vaccine? You can get the new one.

Those who keep up to date with their vaccines and boosters are less likely to get COVID-19 and far less like to be hospitalized or die of it. The Centers for Disease Control and Prevention recommends a reformulated booster for everyone, including children 6 months of age and older, at least two months after completing their primary series or last booster made of the orginal vaccine. Children ages 6 months through 4 years who have completed two doses of the original Pfizer-BioNtech three-dose series may receive the new bivalent Pfizer shot as the third dose in their series and a Pfizer booster at age 5. Children ages 6 months through 5 years who received the Moderna vaccine may receive a bivalent Moderna booster two months after completing that second dose of the orginal vaccine.

Learn more about what boosters are, how they work, and why they’re being recommended.

UPDATED: 12/16/2022

Boosters are common for many vaccines

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.

Because boosters are designed to enhance existing immunity built from the initial series, they are not authorized as substitutes for the initial series in those who have yet to have their first COVID-19 vaccines. Those folks should complete the two-dose series, receiving the original mRNA vaccines, and then get their first bivalent booster two months later.

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.

Some vaccines need boosters more often

Booster A
A. INITIAL SERIES: The first vaccine series triggers naïve B cells to turn into antibody-making B cells (blue) and memory B cells (pink), which stand ready to respond to later exposures. Over time, antibodies and antibody-making cells wane.
Booster B
B. BOOSTER: Memory B cells respond quickly to a vaccine booster dose, producing more antibody-making cells that replenish the protective antibodies.

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 have begun to do the same with vaccines 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.

Photo of a poster that reads: Together We Can Help Stop The Spread of COVD-19

Will boosters protect against omicron variants?

Boosters are still your best protection against severe disease. Until the new omicron-specific formulations were approved, we have had to rely on boosters of the same vaccines made against the original virus identified in January 2020. The initial series can still improve your body’s defenses against COVID-19, but not as effectively as the vaccines did before the virus developed immune-evading mutations. Protection by the original vaccine against omicron and its variants is strong, but it is not permanent.

Omicron is a heavily mutated descendant of the original coronavirus, and the omicron “subvariants” have proven to be highly contagious. The CDC found that during the “omicron period,” an mRNA booster targeting the orginal virus was 91% effective in preventing hospitalization for two months, but that protection waned to 78% by four months. The reformulated boosters are expected to improve protection against the omicron BA.4 and BA.5 subvariants. New variants of the coronavirus could emerge that weaken vaccine effectiveness, and if necessary, the boosters can be reformulated again to target them. Therefore, the CDC urges all to stay up to date with their vaccines, including getting all recommended boosters when eligible.

Immunocompromised people really need boosters

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 Moderna and Pfizer vaccine regimens do not provide as much protection among this population. The CDC has since recommended a three-dose initial series for adults and kids over 12 in this group. The third dose should be given at least 28 days after the second, to be followed by a booster at least two months later, for a total of four shots. Now that bivalent boosters targeting omicron variants are available, the CDC recommends that anyone with a compromised immune system ages 6 months or older receive one of these updated shots at least two months after their completing their initial series or their last booster. For children 6 months through 5 years of age in this group, only the updated Pfizer booster is authorized; those 6 years of age or older may take either the Modern of Pfizer booster. For immunocompromised people who took the Johnson & Johnson vaccine, the CDC recommends a Moderna or Pfizer booster two months after the first shot; those who received the Novavax vaccine are eligible for an updated booster two months after completing their initial series.

Are boosters just as safe as the first doses of vaccine?

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. The single-shot Johnson & Johnson’s vaccine is associated with an extremely rare risk of blood clotting, which can be very serious, among women aged 18-49. As a result, on May 5, 2022, the FDA restricted access to the J&J shot, making it available only to adults who cannot access mRNA vaccines or cannot take them for medical reasons, such as allergic reactions.

When should I get my booster?

The right timing for a booster shot differs from person to person. The CDC says you are up to date on your COVID-19 vaccines if you have completed your primary series with one of the original vaccines and received the most recent booster dose recommended for you. Those recommendations vary, depending on your age and whether or not you are immunocompromised. In general, any adult or child of age 6 months and older is eligible for the new booster — reformulated against omicron — two months after receiving their last shot, whether it was to complete the primary series or was one of the original boosters. Children ages 6 months and older who received the orginal Moderna shots as their primary series are eligible for a Moderna bivalent booster two months after completing that series. Those children of ages 6 months through 4 years who are were taking the three-dose Pfizer vaccine as their primary series may take a bivalent Pfizer vaccine as their third dose in the series and a Pfizer booster at age 5. Those 6 and older may take the Pfizer or Moderna booster. 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, but prefers it be a Pfizer or Moderna mRNA-based vaccine. Anyone ages 12 and above who took the Novavax initial series is eligible for a Pfizer or Moderna bivalent booster two months after completing their initial series. The CDC recommends a more aggressive schedule of initial shots and boosters for individuals with compromised immune systems (see above).

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.

If I’ve gotten one manufacturer’s vaccine, can I be boosted by another’s?

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. There are exceptions for children under 6. However, because of concerns over rare but potentially serious side effects associated with the J&J vaccine, the CDC in December 2021 said that it is preferable in most situations to be boosted, when appropriate, with either the Moderna or Pfizer vaccines.


Will boosters end the pandemic?

They’ll help, but they’re still not the most important factor. Boosters can reduce the spread of COVID-19 in highly vaccinated countries and 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 with the initial series as soon as possible. Providing vaccine protection to the unvaccinated will have more of an 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.

Will we need boosters every year?

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 the troublesome omicron family. 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. Our vaccines will likely need to continue to adapt as the virus changes. Scientists are designing future vaccines that might deliver immunity more effectively through the nose and are researching vaccines that might work across the entire family of coronaviruses that have the potential to cause future pandemics.

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