Why You Need a Booster Even If You’re Vaccinated

Medically Reviewed by Melinda Ratini, DO, MS on February 25, 2022

Vaccines are your best protection against serious illness from COVID-19. If you’re fully vaccinated, the chance you’ll get very sick, end up in the hospital, or die from the disease is much lower.

But studies show COVID-19 vaccines become less effective over time, especially in people ages 65 and older. In general, protection seems to wane about 2 months after a Johnson & Johnson shot and 5 months after your second Pfizer or Moderna shot.

If you’re eligible, the CDC and FDA both recommend a COVID-19 booster shot.

What Is a COVID-19 Booster?

It’s another dose of vaccine. Or, if you get Moderna, it’s half the dose of your first shots. The goal of a booster is to restore and extend your defense against the coronavirus, including variants such as Delta and Omicron.  

But a booster isn’t the same as an “additional dose.” Additional doses are extra vaccines given to people who didn’t build much or any protection after their first shots. You might need one if you have a medical condition or treatment that weakens your immune system.

What Is Fully Vaccinated vs. Optimally Protected?

The CDC considers you fully vaccinated shortly after you’ve finished your primary series of COVID-19 vaccines. For most people, that’s 2 weeks after your second dose of the Pfizer or Moderna vaccines or a single dose of Johnson & Johnson’s vaccine.

You’re optimally protected when you’re up to date on all the recommended COVID-19 vaccines and booster shots. Those guidelines depend on your age, overall health, and when you got your first vaccine.

If you have a moderate-to-severely weakened immune system, you’ll likely need a three-dose primary series of the Pfizer or Moderna vaccines before you get a booster. Your doctor can let you know what’s right for you.

Who Should Get a Booster?

The CDC recommends them for people ages 12 or older. You should get a booster even if you’ve already been sick with COVID-19. For now, they aren’t recommended for kids ages 11 or younger.

You can mix and match your booster. That means it’s OK to get a jab that’s a different brand than your original vaccine. But in most cases, experts suggest you pick an mRNA vaccine. Those are made by Pfizer or Moderna.

Kids ages 12 to 17 can only get the Pfizer vaccine. Adults 18 and older can get either mRNA booster.

There are certain situations where you might consider the Johnson & Johnson booster. That includes if:

  • You’ve had a serious reaction to mRNA COVID-19 vaccines.
  • You’re allergic to an ingredient in mRNA COVID-19 vaccines.
  • You don’t have access to an mRNA COVID-19 vaccine.

COVID-19 vaccines are safe and effective. But talk to your doctor about the pros and cons of each kind. They’ll let you know which booster is right for you.

When Should You Get a Booster?

In general, your jab should come at least 5 months after your second Pfizer or Moderna shot and at least 2 months after a single Johnson & Johnson shot. These guidelines are the same even if you got those vaccines in another country.

It’s OK to get an FDA-approved booster if you were vaccinated outside the U.S. with a different vaccine. But you’ll need to meet the following guidelines:

  • You’re age 12 or older (for Pfizer)
  • You got all recommended doses of a WHO-approved vaccine
  • You got a mix-and-match series of approved vaccines, either FDA or WHO

Talk to your doctor if you have a weakened immune system. You might need a booster sooner than the general public.

Tell your doctor if you’ve tested positive or have symptoms of COVID-19. You’ll need to postpone your booster until you get better. They’ll let you know how long you need to isolate and recover before it’s safe to get your shot.

What Should You Expect After Your Booster?

It’s normal to have some side effects. That’s a natural sign your immune system is working to build protection against the coronavirus.

Your post-shot symptoms might be similar to the ones you had after your first or second vaccine dose. They should go away in a few days.

Common booster side effects include:

  • Redness, pain, or swelling where you got the shot
  • Fever and chills
  • Body aches and headache
  • Tiredness
  • Swollen lymph nodes in your armpit

Call your doctor if you start to feel worse.

Experts don’t advise taking medicine to prevent vaccine side effects. But you can probably use some over-the-counter drugs to feel better after getting a booster. Ask your doctor if it’s safe to take:

  • Acetaminophen
  • Antihistamines
  • Aspirin
  • Ibuprofen

Do not give aspirin to anyone under the age of 19 unless their doctor says it’s OK. It’s linked to a rare but very serious condition called Reye syndrome.

Where Can You Get a Booster?

COVID-19 vaccines and boosters are available for free. But you’ll likely need to make an appointment to get one. You can go to the pharmacy or clinic that gave you your original vaccine or somewhere different.

Bring your CDC COVID-19 vaccine card with you. A health professional will update it with the date and location of your booster. If you don’t have a card, or you lost your old one, ask your pharmacy or state health department how you can get one.

Places that offer a COVID-19 booster might include:

  • Retail pharmacies
  • Hospitals or health centers
  • Walk-up vaccine clinics

If you have trouble getting to a vaccination site, a health care worker might be able to come to you. You can:

  • Contact your doctor or health provider.
  • Call the Medicare hotline at 800-633-4227.
  • Call 211 to find other health services.
  • Get help for an older loved one by calling 800-677-1116.

Check state or local health resources to find a vaccine location near you. You can also visit or call 800-232-0233. People with disabilities can get COVID-19 vaccine info by calling 888-677-1199.

Will You Need More Boosters?

There’s evidence that COVID-19 booster shots might become less effective over time. But it’s not yet clear if that means you’ll need another dose of vaccine in the future.

Drugmakers think we might need coronavirus vaccines every year. Some are working on a single shot that’ll protect against both COVID-19 and the flu. If they’re successful, that could make yearly vaccination easier.

Show Sources

Photo Credit: MarianVejcik / Getty Images


CDC: “COVID-19 Vaccine Booster Shots,” “Vaccines & Immunizations – Interim Clinical Considerations,” “Stay Up to Date with Your Vaccines,” “Johnson & Johnson’s Janssen,” “COVID-19 Vaccines for Moderately to Severely Immunocompromised People,” “Possible Side Effects (COVID-19 vaccination),” “Morbidity and Mortality Weekly Report (MMWR).”

FDA: “Coronavirus (COVID-19) Update: FDA Shortens Interval for Booster Dose of Moderna COVID-19 Vaccine to Five Months.”

Memorial Sloan Kettering Cancer Center: “COVID-19 Booster Shots: Eligibility, Safety, and More You Should Know.”

Journal of the American Medical Association: “Association Between 3 Doses of mRNA COVID-19 Vaccine and Symptomatic Infection Caused by the SARS-CoV-2 Omicron and Delta Variants,” “COVID-19 Vaccine Makers Plan for Annual Boosters, but It’s Not Clear They’ll Be Needed.” 

Yale Medicine: “COVID-19 Boosters: The Latest Advice.”         “Community-Based Testing Sides for COVID-19.” “Find a COVID-19 vaccine near you.”

Stanford Children’s Health: “Reye Syndrome in Children.”

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