What Is the Booster Shot and Why Is It Necessary?

A COVID-19 vaccine booster shot is a vaccine that is administered months after the primary vaccine series. As its name implies, a booster renews or increases the effect of a vaccine. While they seem to be a buzzword in the pandemic, booster shots are not unique to COVID-19. In fact, you have probably received booster shots for other diseases like measles, mumps, and rubella as well as tetanus, diphtheria, and pertussis.

Why Is a Booster Necessary?

Boosters are needed because sometimes vaccines can lose effectiveness over time. When this happens, people become more susceptible to a disease and its symptoms. For COVID-19 specifically, this can mean difficulties breathing, hospitalization, and even death. Essentially, a booster shot helps your immune system regain the defenses it needs to fight off a virus or another pathogen attacking your body.

As with other vaccines, the COVID-19 vaccine can also become less effective down the road. In a joint statement dated August 18, 2021, the US Centers for Disease Control and Prevention (CDC), FDA, and other agency leaders said they were starting to see evidence of waning protection against mild and moderate disease. This was in response to the dominance of the Delta variant, which first appeared in June 2021.

New variants of SARS-CoV-2—the virus that causes COVID-19—form when mutations occur in the virus's genetic sequence. Some mutations are minor. But other mutations can make a variant more transmissible or better at avoiding the effects of a vaccine, among other things.

The three COVID-19 vaccines approved in the US as of May 2022 (Pfizer-BioNTech, Moderna, and Johnson & Johnson vaccines) target a specific protein of SARS-CoV-2 called the spike protein. You are probably familiar with these proteins; they are the telltale broccoli-like projections you see in depictions of the virus.

In October 2021, researchers studying the A.30 variant of SARS-CoV-2 detected mutations that affected the spike protein. These mutations reduced how well vaccines were able to stop this variant from spreading within the body. Fortunately, A.30 was detected in only two countries and did not become widespread. If specific mutations in variants cause future outbreaks, modified booster shots could also be the way to target the new variants.

Who Is Eligible for a Booster?

COVID-19 vaccine boosters are available. And, as of May 2022, CDC recommends them for everyone aged 12 years and older who has received the primary series of the COVID-19 vaccine. The primary series is considered the first two doses of the Pfizer-BioNTech or Moderna vaccine, or one dose of the Johnson & Johnson vaccine. The booster is either another dose of the Pfizer-BioNTech vaccine (for people aged 12 years and older) or the Moderna vaccine (for people aged 18 years and older). CDC also recommends waiting at least five months after the primary series before getting a booster.

When receiving the primary series of the Pfizer-BioNTech or Moderna vaccine, you should get the same product for your second shot. But if you are eligible for a booster and are 18 or older, you can get either the Pfizer-BioNTech or Moderna vaccine, regardless of which one you received in your primary series.

What Is a Second Booster?

As of May 2022, you have to be eligible to receive a second COVID-19 booster. According to CDC guidelines, the groups eligible for a second booster are people aged 50 years and older who had the first booster at least four months ago; people who have moderate or severe immunodeficiency, who are 12 years of age or older, and who got the first booster four months ago; and people who got two doses of the Johnson & Johnson vaccine at least four months ago.

The second booster can only be the Moderna or Pfizer-BioNTech vaccine, so the CDC. The Johnson & Johnson vaccine is not available as a booster, and the Moderna vaccine is available only for adults, ages 18 years or older.

What About People Who Remain Unvaccinated?

The government, top scientists, and infectious disease doctors agree that it's important for everyone to get their COVID-19 shots. Vaccines are effective because they help prevent severe courses and the spread of disease. In tracking COVID-19 data, CDC found that in February 2022, unvaccinated adults ages 18 years and older were five times more likely to end up in the hospital than vaccinated adults. And people who are fully vaccinated and have received an additional or booster dose are the least likely to be hospitalized, according to CDC data from October 2021 to February 2022.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.