How Effective is the Flu Vaccine Every Year?

flu shot calendar
In this article, we will explore how data is collected to track a flu vaccine's effectiveness and the benefits of flu vaccination even if it is not always 100% effective.

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The flu vaccine is one of the most important tools we have to prevent the spread of the flu virus.

Each year, researchers work hard to develop a vaccine that is as effective as possible despite mutations constantly occurring and changing the strains of the viruses.

However, there are many factors that can affect how effective a flu vaccine is in any given year.

In this article, we will explore how data is collected to track a flu vaccine’s effectiveness and the benefits of flu vaccination even if it is not always 100% effective.

We will also discuss some ways that researchers are working to improve the flu vaccine and other ways of preventing the flu.

What is the flu?

The flu is a respiratory infection caused by a group of influenza viruses. It can cause mild to severe illness and in some cases, it can lead to hospitalization or even death.

Influenza type A, influenza type B, and influenza type C are the three types of flu viruses that affect humans. Influenza A and influenza B viruses are the most common strains and both cause epidemics, while type A is also the source of all flu pandemics. Influenza type C is far rarer and usually only causes mild symptoms.

The flu season in the United States is when you are most likely to contract the influenza virus and it runs from October through May with it peaking normally around December through February.

According to a Centers for Disease Control and Prevention (CDC) study published in 2018, it is estimated on average that about 8% of the population gets sick in a typical flu season although these numbers can vary.

What is the flu vaccine?

The seasonal influenza vaccine is a yearly vaccine that helps protect you against the flu. It is made using pieces of inactivated (killed) influenza viruses and it works by causing your body to produce antibodies against the virus.

These antibodies help protect you from infection when you are exposed to the live virus, which is also called an attenuated virus.

The vaccine is usually given as an injection in the arm but it can also be given as a nasal spray and takes about two weeks to become effective. It is recommended that you get it before the start of flu season.

Flu vaccines were first created in the 1940s and soon afterward it was realized that the vaccine needed to be updated every year to keep up with strain mutations.

Since the 1950s, the World Health Organization (WHO) has collected data from over 100 countries around the world to help predict which strains of the virus will be prevalent next flu season.

The manufacturers normally need around six months to manufacture the new vaccine for the following year with the new vaccines becoming available in August through October in the United States.

The current vaccines available in the United States are all quadrivalent, which means that they protect against four of the most likely strains that may be popular for the upcoming flu season.

The two type A vaccine strains and two type B vaccine strains that are incorporated into the 2021-2022 flu season’s annual flu shot include flu A(H1) virus, flu A(H3) virus, flu B/Yamagata lineage virus, and flu B/Victoria lineage virus.

There are also recombinant influenza shots available if you have severe chicken egg allergies.

The current flu vaccine is usually administered as a shot with a needle but there is now a nasal spray flu vaccine available for those who qualify.

Getting a flu vaccine will not give you the flu. However, there are some common side effects after you receive your flu vaccine and these include:

  • Soreness, redness, or swelling at the injection site
  • A low-grade fever for a day or two

Due to the vaccine taking two weeks to be fully effective, you may also get the flu in the time before the vaccine has a chance to take full effect.

The vaccine also does not protect you from the common cold virus which has symptoms similar to the flu which you can still get after receiving your yearly flu vaccination.

Who needs to get the flu vaccine every year?

The CDC recommends that everyone aged six months and older needs to get a flu vaccination every year. If you have had a severe allergic reaction to a previous dose of the flu vaccine or any component of the vaccine, then you need to talk to your doctor about whether there is a vaccine safe for you.

You need to avoid getting the vaccine if you are currently sick as it can cause further complications.

You are also in a high-risk group if you are pregnant and up to two weeks postpartum, you can get the flu vaccine at any time during your pregnancy to help protect yourself and your baby from the virus.

It is very important for elderly adults over 65, young children, and people with weakened immune systems due to medical conditions to get the annual flu vaccine as these conditions put you most at risk of complications from the flu and hospitalization.

Some chronic medical conditions that can also put you at a higher risk for flu complications include:

  • Respiratory illnesses such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis
  • Diabetes
  • Liver disease
  • Kidney disease
  • Heart disease
  • Obesity
  • Cancer and receiving cancer treatments
  • Neurological conditions
  • Nervous system conditions

How is data collected to track a flu vaccine’s effectiveness?

The CDC uses a system called the U.S. Flu Vaccine Effectiveness Network (VE) to track how well the influenza vaccine is working in the United States.

The VE network consists of sentinel providers, which are doctors and clinics that self-report data on laboratory-confirmed influenza cases and vaccination status to the CDC.

The VE network also conducts studies each year during flu season to determine how well the vaccine works.

The CDC then uses this data on influenza activity to make recommendations on how best to use the flu vaccine and how to improve it for future years.

Why get a flu vaccine if it is not always effective?

Even though the flu vaccine is not always effective, it is still the best way to protect yourself from flu activity.

The vaccine is most effective in healthy adults and children over the age of two and is less effective in elderly adults and people with weakened immune systems.

However, even if the vaccine is not 100% effective, it can still help to reduce the severity of symptoms and prevent hospitalization. In its best years, the estimates of vaccine effectiveness range from 40%-60% while in its worst year since data collection started (2004-2005 season), the estimates of flu vaccine effectiveness were only 10%.

The reason for the discrepancies in how effective the flu vaccine is from year to year is because the flu virus mutates constantly changing it from previous seasons and sometimes faster than researchers can keep up with which causes researchers to choose the wrong strain of the virus that will be the most popular circulating strain for the upcoming flu season.

Even in its worst-performing years, you still need to get the flu shot as it can prevent severe symptoms and hospitalizations, and similar to wearing a mask with the coronavirus, it can also prevent you from giving the flu to people who are more at risk of flu complications such as the elderly, unvaccinated people, or immunocompromised people.

In what ways are researchers improving the flu vaccine?

Researchers are constantly working on ways to improve and find the most effective vaccine.

They are working on developing new vaccines that are more effective against a wider range of flu strains and also working on developing new delivery methods, such as a nasal spray vaccine, that can make the vaccine more effective.

In addition, researchers are working on developing new adjuvants, which are substances that can be added to the vaccine to improve its effectiveness.

For example, cell-based flu vaccines now do not require chicken eggs and the same goes for recombinant vaccines.

There are also trials of universal influenza vaccines that will only have to be taken once in your lifetime to provide long-lasting protection for years and do not have to be given annually but none have been approved for production yet.

Finally, researchers are constantly trying to find ways to predict what strain of seasonal influenza will be the most popular the following flu season.

What are other ways of preventing the flu?

In addition to getting an annual vaccination, there are other prevention measures you can utilize to prevent the flu including:

  • Washing your hands regularly and avoid touching your face
  • Avoiding close contact with sick people and stay home if you are sick yourself
  • Cleaning and disinfecting surfaces that are frequently touched, such as door handles, phones, faucet handles, remote controls, and countertops as they can also spread flu germs and infect someone else if they touch them and then touch their mouth, nose, or eyes
  • Coughing or sneezing in the crook of your elbow, sleeve, or in a tissue you throw away and not into your hands
  • Exercising regularly and eating a healthy diet can also help strengthen your immune system and immune response to help fight off viral infections too


The flu is a very serious and contagious disease that can have mild to severe symptoms and lead to hospitalization and sometimes even death.

The best way to protect yourself from the flu is to get a flu vaccine every year.

The seasonal flu vaccine can still help to reduce the severity of symptoms and prevent hospitalization and in its best years, the flu vaccine can reduce your risk of infection by 40%-60%.

However, even in its worst-performing years, you still need to get the flu shot as it can prevent severe symptoms and hospitalizations, and it can also help to prevent you from giving the flu to others who are at high risk for complications from the flu.

If you have any more questions about the flu vaccine or its effectiveness, please talk to your doctor or health care provider.

References, Studies and Sources:



Mayo Clinic

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