What Are Booster Shots?

Medically Reviewed by Michael W. Smith, MD on August 19, 2021

Vaccines help protect us from dangerous viruses and bacteria. Once you've had a shot for a particular disease, you might think you're always safe from it. But that's not necessarily the case.

For some diseases, you need another shot to build strong immunity. For others, your protection wears off over time. And some viruses change, or mutate, over time, making your vaccine less effective.

For most vaccinations, you need more one jab to ward off infection. This extra dose of a vaccine is known as a booster shot.

How Do Vaccine Boosters Work?

A vaccine contains weakened forms of the disease-causing virus or bacteria, or parts of these germs. Or it may be made of an altered genetic "blueprint" for the germ that can make you sick.

The shot triggers your immune system to attack the foreign organism, like it would if you actually got the disease.

This helps your immune system "remember" the disease-causing germ. If you’re exposed to it again, the antibodies can recognize and kill it before it causes harm.

Research has shown that booster shots train your body to recognize the virus or bacteria and defend itself. Depending on the type of vaccine and the manufacturer, you might get a booster weeks, months, or even years after your first shot.

Who Needs Booster Shots?

You may get booster shots as a baby, teen, or adult. You may also need to re-up some vaccinations depending on your lifestyle, travel, or job (for example, if you're a health care worker).

Vaccine boosters that children need include:

Vaccine boosters you may need as a teen or adult include:

Experts recommend that both children and adults get the seasonal flu shot each year. While it’s not 100% effective, it may prevent severe illness. Flu shots are especially important for pregnant women, older adults, and those who have chronic conditions like diabetes or high blood pressure.

During each pregnancy, women need the Tdap vaccine to protect against whooping cough. You usually get it between weeks 27 and 36 of your pregnancy. Along with the flu and Tdap vaccine, health care workers should stay up to date on their hepatitis B, MMR, chickenpox, and meningitis shots.

International travelers may need certain vaccines, depending on their destinations. Antibodies from these vaccines wear off over time, so make sure you're up to date on vaccines for diseases like typhoid. The CDC's Traveler's Health page can help you find out which ones you need.

If you’re not sure which disease you need a booster for, ask your doctor.

COVID-19 Vaccine Booster

Most people who got a vaccine against COVID-19 are already protected against serious illness from the coronavirus.

But even highly effective vaccines often become less effective over time, and the coronavirus vaccines are no exception. Initial evidence on mRNA vaccines like Pfizer and Moderna suggest they start to lose some of their power against infection and serious illness, regardless of variant of the virus (i.e. Alpha, Beta, Delta, etc.).

That’s why the U.S government announced that it will provide booster shots starting Sept. 20, 2021, pending expected CDC and FDA approval. They'll be available nationwide to adults 18 or older who received the Pfizer or Moderna vaccines.

The surgeon general recommends that you time your booster shot for about 8 months after your last dose of mRNA vaccine (Pfizer and Moderna).

In some cases (as in the Pfizer and Moderna vaccines) you may have already had two shots. In others (J&J vaccine) you may have had just one. As of August 2021, there was no booster recommendation for people who got the J&J vaccine, in part because it arrived in the U.S. later than the other vaccines. The U.S. surgeon general’s office said they would give an update on J&J boosters when information becomes available.

WebMD Medical Reference



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Piedmont Healthcare: “Vaccinations and booster shots adults need.”

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University of Iowa Hospitals & Clinic: “Why getting the second dose of your COVID-19 vaccine is so important.”

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CNBC: “Moderna hopes to have Covid booster shot for its vaccine ready by the fall, CEO says.”

U.S. Office of the Surgeon General.

Anthony S. Fauci, MD, director, National Inistitue of Allergy and Inectious Diseases.


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