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Annual Flu Shot Overview

Contributor: Erica S. Shenoy, MD, PhD
5 minute read
A pregnant patient smiles and holds her baby bump while showing a pink bandaid on her left arm after receiving a flu vaccine

The Centers for Disease Control and Prevention (CDC) recommends an annual flu vaccine for anyone aged 6 months or older, with very few exceptions. The flu can cause serious illness, hospitalization, and even death. The flu vaccine reduces the risk of those outcomes. Not only does the flu shot protect you from getting sick, it also helps protect the people in your community. 

“Patients who are older, pregnant, immunocompromised, and very young children are especially at risk of serious outcomes from the flu. By getting the flu vaccines, individuals can reduce their risk of severe illness should they get the flu, and protect those around them,” says Erica S. Shenoy, MD, PhD, chief of Infection Control at Mass General Brigham and an infectious diseases doctor at Massachusetts General Hospital.

Annual flu shot efficacy

Annual flu shots are very effective and each year are reformulated to provide protection against the most likely circulating strains of the virus. Some people who get the flu vaccine, but still come down with the flu, might think that the vaccine doesn’t work well.

“That’s actually not true,” says Dr. Shenoy. “Study after study has shown that people who get the vaccine and become infected with influenza have less severe disease. In fact, last year, the effectiveness of the influenza vaccine was 54% for preventing illness requiring a visit to the emergency department or other outpatient setting, and 71% for preventing symptomatic illness among children and adolescents. This year, the CDC has been describing how the vaccine converts flu from ‘wild to mild,’ and that’s a great description.”

CDC seasonal flu vaccine effectiveness bar graph Source: Centers for Disease Control and Prevention Influenza Division

What is the best time to get a flu shot?

Flu season typically runs from October to May, and in previous years has peaked in the early part of the year. However, in 2022-2023, it came early.

“The CDC recommends getting the flu vaccine in September or October to give you protection for the entirety of the flu season,” says Dr. Shenoy. “Last year the flu season started earlier than usual, and that happened again in the Southern Hemisphere this past summer, so there’s a chance it will come early again this year.” Once you get the vaccine, it takes your body about 2 weeks to build a complete immune response.”

For kids, aged 6 months to 8 years, you need to plan ahead. That’s because kids in this age group may need 2 doses, 4 weeks apart, depending upon prior influenza vaccination.

“It can get a little complicated, so even though I’m an infectious diseases doctor, when I’m being ‘mom,’ I look up the latest recommendations each year.” To learn more about dosing for this age group, you can view the CDC flu vaccine FAQs.

By getting the flu vaccines, individuals can reduce their risk of severe illness should they get the flu, and protect those around them.

Erica S. Shenoy, MD, PhD
Chief of Infection Control
Mass General Brigham

How long is the annual flu shot effective?

The immunity from your flu shot should last for the full duration of the flu season. “You are protected for the current flu season, but each year, you’ll need to get your shot to stay protected,” explains Dr. Shenoy.

An older adult smiles and points to a bandaid on their right arm after receiving a flu vaccine

Is the annual flu shot safe?

The flu shot has been extensively studied and is safe. It’s been used for over 50 years and hundreds of millions of Americans have received it.

Like many vaccines, you can have some side effects, such as soreness from the shot, a headache, fever, nausea, and muscle aches. These are generally mild and go away in a few days. Despite these symptoms, you can’t get the virus from the vaccine.

Erica S. Shenoy, MD, PhD


Chief of Infection Control
Mass General Brigham