Does the Flu Shot Work?

The flu (influenza) is a respiratory infection caused by influenza viruses. These viruses are very contagious and spread easily through the air and on surfaces. They can enter your body through your nose, eyes, or mouth — where they attack your nose, throat, and lungs.

The flu can affect anyone — even young, healthy people. For some people, influenza viruses may only cause mild illness. But other people can become very sick.

The last couple of years have seen particularly rough flu seasons. For example, in 2024-25, the U.S. had the highest death rate among children since 2004, and 89% of the population wasn’t vaccinated. Also, nearly 11% of people hospitalized for flu-related illness had no underlying health conditions. The 2025-26 flu season was just as bad, but with an earlier peak.

Does the flu shot work to prevent the flu?

Yes. The flu shot is the most powerful defense we have against seasonal flu. That’s why experts say almost everyone over the age of 6 months should get a flu vaccine every year.

The CDC estimates that the flu vaccine prevented about 10 million flu illnesses and 12,000 deaths in the 2024-25 flu season. The flu shot also kept about 180,000 people out of the hospital in that same period.

Of course, the flu shot isn’t perfect. Its effectiveness varies from year to year, depending on how well the vaccine matches the viruses spreading in the community. Age also matters: The flu shot might not be as effective in children under age 2 and adults over age 65. Between those age ranges, the flu shot is up to twice as effective in children compared with adults.

Vaccine effectiveness is a measure of how well the vaccine protects people from the flu, compared to people who aren’t vaccinated.

For example, in 2023-24, the flu vaccine was 44% effective. This means people who got the vaccine were 44% less likely to get sick with the flu than people who didn’t get the vaccine.

Here’s how well the flu vaccine has worked in recent years in terms of effectiveness and flu prevention.

How does the flu shot work to protect you?

The flu shot will make you less likely to catch the flu, but that’s not the only benefit. The flu shot can also:

• Make your flu symptoms less severe if you do get the flu

• Lower your chances of hospitalization or death if you get the flu

• Reduce the spread of the flu in your community

• Protect pregnant people and babies, who are especially vulnerable to the flu

• Reduce flare-ups of other health conditions — like asthma, chronic obstructive pulmonary disease (COPD), and heart disease — for people who have them

How effective is the flu shot?

Let’s look at the numbers. As mentioned above, how well vaccines work tends to vary year to year. But research shows that vaccination is beneficial in several ways, even if you do get the flu:

• Requiring medical attention is half as likely.

• Hospitalization is 42% less likely overall.

• Hospitalization is 52% less likely for children.

• Hospitalization is 40% less likely for pregnant people.

• The need for mechanical ventilation (breathing machine) is 55% less likely.

The flu shot can also protect you if you have a chronic health condition like heart disease. That’s because the flu can be very hard on your heart and lungs. In some cases, it can even trigger an asthma attack or heart attack.

Studies suggest that flu vaccination can do the following:

• Lower your risk of having a major heart-related event (such as a heart attack or stroke) by 34% to 45% after getting the flu

• Prevent between 59% and 78% of asthma attacks that otherwise would have led to emergency room visits and hospitalizations

Finally, getting a flu shot can protect you against life-threatening flu complications such as:

• Pneumonia

• Worsening of lung issues or lung conditions 

• Heart inflammation (myocarditis)

• Brain inflammation (encephalitis)

• Muscle inflammation (myositis) 

• Muscle breakdown (rhabdomyolysis)

• Sepsis 

• Death

How the flu shot is made

Three different types of influenza viruses can cause symptoms in humans: A, B, and C.

Every year, three to four influenza A and B viruses are included in the flu shot (C is less common and doesn’t cause pandemics).

Production of the flu shot is a year-round process. Throughout the year, researchers around the world monitor influenza outbreaks in their communities. They watch to see which strains of flu are making people sick and how well the current flu shot is working. Based on this information, the World Health Organization (WHO) and its partners recommend which influenza viruses should be included in next year’s flu shot.

After the WHO makes its recommendations, vaccine production begins. The vaccine is made by growing the live viruses in a laboratory, then inactivating them to kill or weaken them. 

Recombinant vaccines include a synthetic version of one of the flu virus’ proteins instead.

After the flu shot, your immune system will see the virus or protein as a threat and make special proteins — antibodies — to defend you. Antibodies stick to foreign invaders in your body and tell your immune system where to attack.

It takes your body a couple of weeks to build immunity. So, the flu shot is most effective about 2 weeks after you get it, and it offers protection for 6 months or longer.

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Written by:

Mandy Armitage, MD

Mandy Armitage, MD, has combined clinical medicine with her passion for education and content development for many years. She served as medical director for the health technology companies HealthLoop (now Get Well) and Doximity.

Edited by:

Sophie Vergnaud, MD

Sophie Vergnaud, MD, is the Senior Medical Director for GoodRx Health. A pulmonologist and hospitalist, she practiced and taught clinical medicine at hospitals in London for a decade before entering a career in health education and technology.

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