A woman pulls up her sleeve while a doctor injects a flu shot into her upper arm. Share on Pinterest
ozgurcankaya / Getty Images

The flu is a respiratory illness that’s caused by influenza viruses. In many parts of the world, these viruses cause seasonal epidemics during the colder months of the year.

The flu vaccine helps to protect you from getting sick with the flu. It’s made based on the strains that scientists predict will be most common during the upcoming flu season. Because of this, it can vary in effectiveness from year to year.

This article will explore the benefits of the flu vaccine, how effective it is, and what may influence the vaccine’s effectiveness.

Before we discuss the effectiveness of the flu vaccine, let’s break down its various benefits.

The flu vaccine reduces your risk of getting sick

The main benefit of the flu vaccine is that it can prevent you from becoming ill with the flu. The Centers for Disease Control and Prevention (CDC) estimates that for the 2019–2020 flu season, vaccination prevented 7.52 million illnesses in the United States.

While the flu typically causes mild to moderate symptoms in many people, getting sick can still be unpleasant and make it difficult for you to go about your daily life. It can also lead to increased doctor’s visits and missed days of school or work.

Helps prevent serious illness

The flu can lead to potentially serious complications like pneumonia or the worsening of underlying health conditions. Some people are more at risk for serious illness from the flu. This includes:

Getting the flu vaccine may help protect these higher-risk individuals from contracting the flu and potentially becoming very ill.

In fact, the CDC estimates that flu vaccination prevented 105,000 hospitalizations during the 2019–2020 flu season.

May reduce the severity of flu symptoms

Some people who get the flu vaccine still become sick with the flu. However, there’s evidence that symptoms may be less serious in these individuals.

For example, a 2017 study evaluated how vaccination impacted flu severity in adults who were hospitalized with the flu. It found that vaccination was associated with lower intensive care unit (ICU) admission and shorter hospital stays.

A 2020 study investigated pediatric emergency room visits and hospitalizations during the 2018–2019 flu season. It found that flu vaccination reduced influenza-related emergency room visits and hospitalizations by 40 to 60 percent.

Helps protect people in your community

When many people within a community receive the flu vaccine, it can help prevent influenza viruses from effectively circulating within that community.

This can be an extra layer of protection, particularly for those at risk of serious illness.

Flu vaccine effectiveness can change from year to year. At the time of writing this article, the 2020–2021 flu season is underway. In the coming months, scientists will begin to get estimates of the effectiveness of the 2020–2021 flu vaccine.

However, the CDC does currently have information about the estimated effectiveness of the vaccines from past flu seasons.

The chart below shows the effectiveness of the flu vaccine for the previous five flu seasons.

Flu seasonPercent vaccine effectiveness
2019–202039 percent
2018–201929 percent
2017–201838 percent
2016–201740 percent
2015–201648 percent

Does flu vaccine effectiveness differ across age groups?

It’s possible that there can be some variance in the effectiveness of the vaccine between different age groups. The CDC tracks this data as well as data on the flu’s overall effectiveness.

Here’s a look at the effectiveness estimates for all vaccine types across different age groups from the past five flu seasons, according to the CDC.

Flu seasonAll ages6 months to 8 years9–17 years18–49 years50–64 years65 years and older
2019–202039 percent34 percent40 percent34 percent40 percent39 percent
2018–201929 percent48 percent7 percent25 percent14 percent12 percent
2017–201838 percent68 percent32 percent33 percent30 percent17 percent
2016–201740 percent57 percent36 percent19 percent40 percent20 percent
2015–201648 percent51 percent59 percent52 percent26 percent42 percent

When looking at vaccine effectiveness studies, it’s important to remember that the results can be influenced by several factors. These factors can include the population studied, the specific flu season, and how the study was conducted.

There are several different types of flu vaccines. Many of them are given as an injection. These vaccines contain inactivated (“dead”) flu virus or only single viral proteins.

The nasal spray vaccine is a flu vaccine that’s sprayed into your nose. It’s made up of virus that’s been weakened so it can’t establish an infection. It’s known as FluMist or the live-attenuated influenza vaccine (LAIV).

In previous years, the nasal spray vaccine wasn’t recommended. The reason for this was because it had less effectiveness in children against certain types of influenza viruses.

However, there have been recent improvements in the manufacturing of this vaccine and some data indicates that the effectiveness of the nasal spray vaccine is now similar to that of the flu shot.

Because of this, the nasal spray vaccine has been recommended since the 2018–2019 flu season. In fact, the CDC states that there’s no preference for one type of vaccine over another for the 2020–2021 flu season.

Previously, we discussed that the effectiveness of the flu vaccine can vary from year to year. Let’s take a deeper dive into why this is the case.

Flu strains chosen for the vaccine

Influenza viruses are constantly changing. Because of this, the influenza strains that were common during last year’s flu season may not be as prevalent for the current flu season.

Early each year, scientists meet to select the strains to include in the flu vaccine for countries in the northern hemisphere, including the United States. These meetings are held early in the year because vaccine producers need time to make the vaccine so it’s ready for the beginning of the flu season in the fall.

Vaccine strains are chosen based on what current surveillance data predicts may be the most common strains for the upcoming flu season. However, sometimes the selected strains don’t match well with the strains that actually end up being most prevalent during a flu season.

When this happens, vaccine effectiveness can be low. However, if the strains selected are a good match, vaccine effectiveness is higher.

Influenza virus subtype

Flu vaccines protect against two types of influenza virus: influenza A and influenza B.

Influenza A viruses are very diverse and are further divided into different subtypes. Two subtypes that you may be familiar with are H1N1 and H3N2.

Typically, flu vaccines give good protection for influenza B and H1N1 strains. However, they provide less protection against H3N2 strains. This is due to the various genetic changes that can commonly happen in H3N2 viruses.

A 2016 review looked at 142 studies on flu vaccine effectiveness by flu type. It found that:

  • Vaccine effectiveness against H3N2 strains was only 33 percent.
  • Meanwhile, vaccine effectiveness was 54 percent and 61 percent against influenza B and H1N1 strains, respectively.
  • When H3N2 vaccine strains were a good match, vaccine effectiveness was still only 33 percent. When vaccines weren’t a good match, effectiveness dropped to 23 percent.

So, if H3N2 strains are more common during a flu season than other types of influenza, the effectiveness of the vaccine can be lower.

Individual factors

Individual factors, like your age and overall health, can also impact vaccine effectiveness. For instance, older adults tend to have a weaker response to influenza virus vaccination.

To help with this, a high-dose flu vaccine is available for adults aged 65 and older.

Large-scale studies of vaccine effectiveness in this population have found that the high-dose vaccine is slightly more effective than standard dose vaccines.

The current recommendation for the 2020–2021 flu season is that all individuals aged 6 months and older receive a flu vaccine. No one type of vaccine is recommended over another.

Getting the flu vaccine is particularly important for people that are at an increased risk of severe illness from the flu. This includes:

Flu vaccination and COVID-19

Getting a flu vaccine is particularly important during the COVID-19 pandemic. There are a couple of reasons for this:

  1. A flu vaccine can help protect you from getting sick with the flu, which has similar symptoms to COVID-19.
  2. The benefits of flu vaccination can help keep you out of the doctor’s office and hospital, which in turn helps to conserve medical resources for those sick with COVID-19 or other health conditions.

Generally speaking, a flu vaccine should protect you through the current flu season. You’ll need to receive another flu vaccine for the next flu season. There are several reasons why a flu vaccine is only effective for one flu season.

First, influenza viruses change constantly. Because of this, the strains included in this year’s flu vaccine may not be the most common strains during next year’s flu season.

Second, the level of protection (immunity) generated by the flu vaccine wanes over time.

For example, a 2019 study found that, across seven flu seasons, every additional 28 days after vaccination was associated with a 16 percent increase in the likelihood of testing positive for flu.

When is the best time to get vaccinated?

The CDC recommends getting your flu vaccine by the end of October each year. However, getting vaccinated later than this can still provide valuable protection.

It’s generally a good rule of thumb to avoid getting vaccinated early, like in July or August. This is because the immunity generated by the vaccine declines over time. So, you could be at an increased risk for contracting the flu later in the season if you get the vaccine too early.

The flu vaccine helps to protect you from getting sick with the flu. It can also reduce the seriousness of your illness if you become sick. Additionally, it can help prevent influenza viruses from circulating widely in your community.

Flu vaccine effectiveness changes from year to year due to several factors. Its effectiveness can be affected by viral evolution and by the subtype of influenza that’s predominant in a given year. Individual factors like your age and overall health can also play a role.

Because influenza viruses are always changing and the immunity generated by the flu vaccine wanes over time, you’ll need to get a new flu vaccine every year, preferably by the end of October.