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. For some people, influenza viruses may only cause mild illness. But other people can become very sick.  

The flu is a serious problem worldwide. It infects up to 8% of people in the U.S. every year, and it causes tens of thousands of deaths. In fact, it’s still one of the top 10 causes of death in the U.S. The flu can affect anyone — even young, healthy people.

Many people think that you don’t need to get vaccinated if you’re young and healthy. But the flu vaccine is for everyone, and it’s safe. Everyone over 6 months old should get a flu vaccine every year. It’s the best way to protect yourself and your community from the deadly risks of influenza. 

Does the flu shot prevent the flu?

Yes. The flu shot is the most powerful defense we have against seasonal flu. The CDC estimates that the flu vaccine prevented about 1.8 million flu illnesses and 1,000 deaths in the 2021-2022 flu season. The flu shot also kept about 22,000 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. 

Your personal characteristics can affect how well the flu shot works for you. For example, the flu shot seems to be most effective in people younger than 65, especially adult women. It may be slightly less effective in people who carry excess weight or have other health conditions. 

The CDC monitors vaccine effectiveness every year. They consider how well the vaccine matches what’s in the community along with personal characteristics like age, underlying conditions, and vaccination history. 

Effectiveness is defined as the “percent reduction in the frequency of flu illness among vaccinated people compared to people not vaccinated.” It’s a measure of how well the vaccine protects people from the flu. 

For example, in 2022-23, the flu vaccine was 54% effective. This means people who got the vaccine were 54% 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.

These numbers are impressive, but they don’t tell the whole story. That’s because the flu shot does more than just protect you from getting sick. Even in years when the flu shot is less effective, it still does more good than harm. 

Quiz: Test your flu shot knowledge

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, COPD, and heart disease) for people with these conditions

People who get the flu after being vaccinated are 37% less likely to be hospitalized and 23% more protected from a serious outcome — like ICU admission or death — than they would be if they were not vaccinated.

And in a 2022 study, children hospitalized with the flu were 75% less likely to die if they’d had the flu vaccine than if they did not.

The flu can be dangerous for people who are pregnant. Getting the flu shot makes pregnant people 40% less likely to be hospitalized if they catch the flu.

The flu shot can also protect you if you have a chronic health condition like heart disease. The flu can be very hard on your heart and lungs. In some cases, it can even trigger a heart attack. That means the flu shot may actually lower your risk of having a heart attack. 

Asthma can also cause problems if you catch the flu. In a large meta-analysis, the flu vaccine prevented 59% to 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

• Heart inflammation (myocarditis)

• Brain inflammation (encephalitis)

• Muscle inflammation (myositis) 

• Muscle breakdown (rhabdomyolysis)

• Sepsis 

• Death

Why not get the flu shot?

Even though the flu shot works well, many people still do not get vaccinated. In fact, only about half of the U.S. population gets the flu vaccine. There are a number of reasons for this. 

For one thing, some people might be worried about how to pay for it. Others might not know where to get one. Still others may be worried about vaccine myths, or they just don’t like getting a shot every year. 

But flu vaccines are available to everyone. Online resources can help you find a flu shot in your area, and a free or low-cost vaccine might be an option for you. 

Scientists are working on a universal flu vaccine that could last much longer and protect against many influenza viruses (instead of just a few). But, at the moment, you still need to get a new flu shot every year.

Will the flu shot work this year?

It’s impossible to say exactly how well this year’s flu shot will work. That’s because flu viruses change all the time. The viruses included in this year’s flu shot were chosen in March — 6 to 8 months before flu season actually began — so the viruses have had lots of time to evolve. 

The flu shot works best when the vaccine is well matched with the viruses in the community. If the viruses haven’t changed a lot between March and September, the current flu shot will be a good match. And if it’s a good match this year, the flu shot will work well. 

During flu season, the CDC watches closely to see how well the flu shot works. It shares this information with healthcare providers and the public. This information is also used to plan next year’s flu shot. 

How the flu shot is made

The first influenza vaccines were created in the late 1930s and early 1940s. 

Today, making 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. 

Every February, the World Health Organization (WHO) and its partners make predictions about the upcoming flu season. They recommend which influenza viruses should be included in next year’s flu shot. 

Three different types of influenza viruses can cause symptoms in humans: A, B, and C. Only influenza A and B are included in the annual flu vaccine: 

• Influenza A is a common flu virus that can infect humans and animals. There are many different versions of influenza A viruses (also called subtypes, or strains). Two of these strains — H1N1 and H3N2 — spread regularly during flu season. An H3N2 flu season can be especially deadly. Influenza A can change and evolve quickly, which makes it more likely to cause pandemics. 

• Influenza B is another common influenza virus. It only infects humans. In general, influenza B is less severe than influenza A. It has not caused pandemics in the past. 

• Influenza C is less common than A or B and only causes mild illness. It’s not included in the yearly flu vaccine and does not cause pandemics.

Every year, three to four influenza A and B viruses are included in the flu shot. 

After the WHO makes its recommendations, vaccine production begins. The vaccine is made by growing the live viruses in a laboratory. The viruses are then inactivated (attenuated). These processes kill or weaken the viruses. The killed and weakened viruses can’t make you sick. 

The flu shot contains these dead or weakened viruses. Even though the viruses can’t hurt you, your immune system still sees them as a threat and makes special proteins — antibodies — to defend you. Antibodies stick to foreign invaders in your body and tell your immune system where to attack. 

After you have the flu shot, it takes your body about 2 weeks to make these antibodies. The antibodies circulate in your blood and protect you from the flu for up to 6 months. 

Who should get a flu shot?

Everyone over the age of 6 months should get a flu vaccine every year. But talk with your healthcare provider before getting the flu shot if:

• You are currently sick with a fever. This does not mean you can’t get the flu shot. But your healthcare provider may recommend you wait until your current symptoms have passed.

• You have a history of Guillain-Barré syndrome (GBS). 

• You have had a severe allergic reaction in the past to any of the flu shot ingredients. 

When to get a flu shot

Flu season usually starts in September, and it can go until as late as May the following year. Ideally, you should get your flu shot in the fall, just before flu season begins. But you can get the flu shot at any time while influenza viruses are circulating. 

The flu shot is most effective about 2 weeks after you get it, and it offers protection for 6 months or longer. 

This year, with COVID-19 still spreading widely, it’s especially important to get the flu shot. It’s possible to have COVID infection and the flu at the same time. So it’s a good idea to be vaccinated against both. 

Does the flu shot protect against the common cold or COVID? 

No. The flu shot only protects you against the three to four influenza viruses included in this year’s vaccine. It will not protect you from the virus that causes COVID. Nor will it protect you from the many other types of viruses that can cause cold and flu-like symptoms, like adenoviruses, respiratory syncytial virus (RSV), and other types of coronavirus. The flu shot will also not protect you against bacterial infections.

In addition to getting the flu shot, use good preventive habits during the flu season: 

• Wash your hands frequently.

• Avoid touching your face.

• Avoid people who are sick.

Is the flu vaccine safe during pregnancy?

The flu vaccine is safe and effective during pregnancy. If you’re pregnant, you should get a flu vaccine. Being pregnant increases your risk of being hospitalized or dying from the flu. The flu can also cause pregnancy complications like preterm labor and preterm birth.

The flu vaccine protects both the mother and the growing baby during pregnancy. It will also protect the baby for up to 6 months after birth. This is especially important because babies younger than 6 months are too young for the flu shot. And babies can get very sick and even die from the flu. 

Can the flu shot make you sick?

No. The flu shot is made from dead or weakened virus fragments. It can’t give you the flu. 

Some people may have side effects from the vaccine for 1 to 2 days. The most common side effect is soreness or swelling at the injection site. 

In rare cases (less than 1%), people can have flu-like symptoms, like a low fever, muscle aches, or fatigue. These symptoms are a normal response to your body making antibodies. They do not mean you’re getting the flu.

Why trust our experts?

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:

Alex Eastman, PhD, RN

Alex Eastman, PhD, RN, is a California-based registered nurse and staff medical editor at GoodRx, where he focuses on clinical updates and Latino health.

Reviewed by:

Kerry R. McGee, MD, FAAP

Kerry McGee, MD, FAAP, has over a decade of experience caring for babies, children, and teenagers as a primary care pediatrician. She has a special interest in adolescent health, particularly in adolescent mental health.

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