What Are Booster Shots?

Medically Reviewed by Brunilda Nazario, MD on April 15, 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 and malaria. 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

COVID-19 vaccines help your immune system build antibodies against this highly contagious virus. Research shows these vaccines are safe and effective.

Currently in the U.S., two COVID-19 vaccines are authorized by the FDA. Both require boosters. Just one shot of either gives you some immunity. But a second dose gives you the best protection.

The recommended booster schedules are:

  • Pfizer-BioNTech: Two shots given 21 days apart.
  • Moderna: Two shots given 28 days apart.

Things to know about the booster shot:

  • The two types of vaccines are not interchangeable, so your second dose should come from the same manufacturer as your first one.
  • Get your second shot as close as possible to the recommended time.
  • If you can't get the second dose in time, you can get it up to 6 weeks after the first one.

You're more likely to have mild side effects, like fatigue and a headache, after the second dose of the vaccine than after the first one. But these shouldn’t last more than 1-3 days. Serious allergic reactions are rare. If you have one, get medical help right away.

As scientists learn more about the virus, they’ll understand more about how long immunity from the COVID-19 vaccine will last. Pfizer and Moderna have said people will probably need a third shot to keep their immunity, and you could need annual shots, much like the flu vaccine. 

Ask your doctor if you’re eligible to get the COVID-19 vaccine. If you've ever had a reaction to any vaccine, tell your doctor before you get a COVID-19 vaccine.

WebMD Medical Reference



CDC: “Understanding How Vaccines Work,” “What Vaccines are Recommended for You,” “Different COVID-19 Vaccines,” "Travelers' Health."

UpToDate: “Patient education: Vaccines for infants and children age 0 to 6 years (Beyond the Basics).”

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

Children’s Hospital of Philadelphia: “Questions and Answers about COVID-19 Vaccines.”

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

WHO: “How do vaccines work?”

Memorial Sloan Kettering Cancer Center: “The Second Dose of a COVID-19 Vaccine: Side Effects, Why They Happen, and How to Treat Them.”

Cleveland Clinic: "Vaccine FAQs: How Are Vaccines Developed, and How Do They Work?"

Reuters: “U.S. preparing for 1-year COVID-19 booster shots; Pfizer chief sees need.”

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


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