Understanding Flu Vaccine Effectiveness

The flu vaccine reduces risk of illness, hospitalization and death

The flu vaccine offers protection against four common strains of the flu. The flu vaccine's effectiveness is 40%–60% during years when the most prevalent flu strains match the strains in the vaccine.

In reality, flu vaccine effectiveness has ranged from 29%–48% over the five seasons prior to 2020–2021. That means getting the vaccine every year reduces a person’s risk of contracting the flu by up to 48% in recent years.

The vaccine also reduces the likelihood of severe illness, hospitalization, and death. Because of this, the Centers for Disease Control and Prevention (CDC) recommends that everyone ages 6 months and over get the flu vaccine (with very rare exceptions).

There are nine different flu vaccines available for the 2021–2022 flu season. All of them protect against the same four strains of flu. Here’s what you should know about flu vaccine effectiveness and types of flu vaccines. 

Woman getting a flu shot

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Flu Vaccine Effectiveness for 2021–2022 Season

Each year, the flu vaccine is designed to protect against the strains of the flu that scientists believe will be most common. However, the effectiveness of the flu vaccine depends on which flu strains are most prevalent and how they compare to the strains that the vaccine protects against. 

Because of this, flu vaccine effectiveness varies from year to year. Scientists don’t yet have estimates for the effectiveness of the flu vaccine for the 2021–2022 season. In addition, in the 2020–2021 flu season the CDC didn’t estimate the effectiveness of the vaccine because transmission of the flu was historically low, likely due to COVID-19 precautions that also protect against the flu.

During Past Flu Seasons 

The CDC tracks seasonal flu vaccine effectiveness each year. Over the past 17 years, the highest effectiveness was in the 2010-2011 season when it was 60% effective.

Here are the effectiveness rates over the past five seasons for which data are available:

  • 2015–2016: 48% 
  • 2016–2017: 40%
  • 2017–2018: 38%
  • 2018–2019: 29%
  • 2019–2020: 39%

Reasons to Consider Getting a Flu Vaccine 

The flu vaccine (flu shot or nasal spray vaccine) significantly reduces the risk of contracting the flu. Consider the 2019–2020 season: People who had the vaccine were 39% less likely to get the flu than those who did not.

In addition to preventing illness, the flu vaccine also reduces the risk of hospitalization and death. Here’s what researchers have found about the flu vaccine:

  • Getting the vaccine reduces the risk of intensive-care unit (ICU) hospitalization by 26% and the risk of death by 31%, according to a 2021 study.
  • Among adults who are hospitalized for the flu, people who are vaccinated are 59% less likely to need intensive care unit (ICU) care, according to a 2018 study. 
  • People with heart disease who get the vaccine are less likely to experience cardiac events.
  • People with diabetes and chronic lung disease who get the vaccine are less likely to be hospitalized for those conditions. 
  • Pregnant people who get the vaccine are 40% less likely to be hospitalized for flu than pregnant people who did not get the vaccine.

In addition to protecting you, getting the flu vaccine can keep you from contracting the flu and passing it to others, including infants and the elderly, who are at higher risks for complications.

Pregnant people who get the vaccine help protect their baby from the flu (passing on the protective antibodies developed by the vaccine), which can be valuable during the first six months of life when a child is not able to be vaccinated.

Flu Vaccine Efficacy Factors 

There are a number of factors that influence how effective the flu vaccine is every year. The most important is strain matching—whether the strains that the vaccine protects against match the most widely circulating flu strains during a given season.

In addition to the construction of the vaccine, there are more personal factors that influence vaccine effectiveness.

Strain Matching

To decide which strains are covered by the flu vaccine, data on flu infections are collected from around the globe. Using that, the World Health Organization (WHO) meets twice a year to decide which strains to focus on in the vaccines.

For the Northern Hemisphere, the WHO makes a recommendation in February. In the United States, the Food and Drug Administration (FDA) has the final say in which strains are included in the vaccine.

The decision about which strains to include is based on projections that are made using past data. This gives time to manufacture the vaccine.

Vaccine Timing

It takes two weeks for the flu vaccine to become optimally effective. Because of that, the CDC recommends that all people get the flu vaccine by September or October, before flu infections become widespread.

Getting the vaccine too late can leave you exposed to early infection from the flu. On the other hand, the vaccine protection begins to wane after several months. Adults ages 65 and older are generally not vaccinated early (July or August) due to this.

The CDC recommends the vaccine (even later in the season) for people who didn't get vaccinated in September or October, since flu can spread throughout the spring.

Age

CDC data show that within a given year, the protection that the flu vaccine offers can vary based on age, due to immune changes over time. There’s no way to predict which age groups the flu vaccine will be most or least effective for in a given season.

Some research has shown that vaccines that cause a stronger immune response are best for people 65 and older. There are two vaccines approved only for this population: one contains a larger dose of the vaccine and the other contains an ingredient to boost immune response. Still, the CDC says that older people can get any type of flu vaccine.

Health

Vaccines rely on a person's immune system responding to the vaccine by producing antibodies, which are proteins that have specific targets, in this case on the influenza virus.

If a person has an impaired immune system due to a health condition, they may not mount the desired immune response, reducing the effectiveness of the vaccine. There are a wide variety of health conditions that can impair your immune system response.

This is one reason that everyone should be vaccinated. By having less risk of getting and spreading the flu, they protect those who have impaired immunity and, therefore, can't benefit as much from being vaccinated themselves.

Types of Flu Shots and Vaccines 

There are nine types of flu vaccines (shots and nasal spray) available for the 2021-2022 season. All nine protect against the same four strains of flu. Because they protect against four they are called quadrivalent vaccines.

The CDC doesn’t recommend one type of vaccine over the others: the vaccine that’s available is the one you should get. However, some people, including people under the age of 2, over the age of 65, who have allergies or who are pregnant have special considerations and should talk to their doctor about which vaccine is best.

Adults 65 and Up

The CDC does not recommend a specific vaccine for adults over age 65. However, some studies indicate that a higher-dose vaccine or a vaccine that uses an adjuvant (a substance that enhances the body’s immune response to an antigen) to produce a stronger immune response are more effective for this population. These are:

  • Fluad Quadrivalent: Uses an ingredient to elicit a stronger immune response and is approved for people ages 65 and older 
  • Fluzone High-Dose: Has a larger dose of antigen to produce a larger immune response and is approved for people ages 65 and older 

If You’re Allergic 

Many flu vaccines are made using eggs or egg protein, and present an allergy risk for some people. The two egg-free vaccine options are:

  • Flublok Quadrivalent: Approved for people ages 18 and older
  • Flucelvax Quadrivalent: Approved for people ages 6 months and older

If You’re Pregnant 

Pregnant people can get any flu vaccine except for FluMist, which contains a live virus.

Nasal Spray Option 

FluMist Quadrivalent is a vaccine administered via a nasal spray. It is approved for people ages 2 through 49. However, it’s the only vaccine that contains a live, weakened virus, and it should not be used by pregnant or immunocompromised people. Talk to your healthcare provider about whether FluMist is safe for you.

Flu Vaccine for Babies

There are five flu vaccines approved for babies 6 months and older. They are:

  • Fluzone Quadrivalent
  • FluLaval Quadrivalent
  • Fluarix Quadrivalent
  • Afluria Quadrivalent
  • Flucelvax Quadrivalent

Flu Vaccine Effectiveness Over Time 

The flu vaccine needs two weeks to become fully effective, and protection is thought to last for at least six months. But a person's antibody levels in response to the vaccine can decrease over time, and different flu strains may be circulating.

Because of that and because each year the vaccine is tailored to the most concerning flu variants, people should get a flu vaccine annually. 

Flu Vaccine Side Effects

Side effects of the flu vaccine are usually mild and go away without treatment in a few days. Common side effects include:

  • Soreness, redness, and swelling where the shot was given
  • Headache
  • Fever
  • Nausea
  • Muscle aches
  • Fatigue

Rarely, allergic reactions occur, often within minutes of the vaccine being administered. If you experience swelling, racing heart or trouble breathing, seek medical attention immediately. Extremely rarely (no more than one or two cases per million people who receive the flu vaccine) a person may develop Guillain-Barré syndrome, an autoimmune disease.

Summary

The flu vaccine can prevent illness, hospitalization, and death. The CDC recommends that everybody aged 6 months and older get the vaccine every year (with very rare exceptions). The effectiveness of the vaccine varies since the vaccine only protects against certain strains of flu. Over the past five years, the effectiveness of the flu vaccine has ranged from 29%–48%. 

A Word From Verywell

Deciding whether to vaccinate can be a big decision. Although the flu is a common illness, it can be deadly, particularly for people over the age of 65. The flu vaccine isn’t 100% effective, but it does significantly reduce the risk of illness, hospitalization, and death.

If you have questions about the vaccine, talk to your doctor about whether it is right for you or your children. 

Frequently Asked Questions

  • When did the flu shot first come out?

    The flu vaccine first became available in the 1930s, and it became widely available in 1945.

  • Has flu vaccine efficacy increased since the 20th century?

    Flu vaccine effectiveness varies from year to year. It depends on how well the strains in the vaccine match the strains that are most prevalent in a given year.

  • Is it safe to get the flu and COVID-19 vaccines together?

    Yes, the CDC says that the flu vaccine and COVID-19 vaccine, including boosters, can be administered at the same time.

  • What is the flu shot made of?

    The flu shot is made with either killed or weakened flu virus. The shots also contain inactive ingredients, including preservatives.

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14 Sources
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