How Flu Shots Work and Why They Sometimes Don't

Flu shots take many months to manufacture, so the formula for the vaccine is developed during the previous year’s flu season. Three or four different strains of the influenza virus are chosen to be included in the vaccine to (hopefully) provide as much protection to as many people as possible. Researchers look at the strains of influenza that are circulating and how they have been mutating to try to determine the strains that are most likely to be causing illness during the following flu season.

Once the strains are chosen, manufacturers begin developing the vaccine. In fact, some manufacturers may begin to do this up to a month before the new formula is announced so they will be prepared and have enough time to make adequate amounts.

The flu vaccine takes at least six months to make, so it is no small feat to get it prepared for the beginning of flu season each year.

How the Flu Virus Changes

The various influenza viruses mutate constantly, and this is one reason that yearly flu shots are necessary. The virus can change in two different ways. A slight change is called a “drift,” while a major change it is called a “shift.” Interestingly, only influenza A viruses can mutate by “shifts.”

Why the Flu Shot Doesn’t Always Work

Typically each year, one or two of the viruses in the flu shot are updated to anticipate mutations. However, if a major “shift” occurs, or the virus mutates to a different form than what researchers anticipated, the shot may not cover some of the circulating viruses. If there are viruses circulating that are not covered by the vaccine, you may still get the flu even if you had a flu shot.

The good news is that the antibodies your body produces to fight the viruses in the flu shot are typically able to provide some resistance to mutated versions of that virus. Even if it does not prevent the illness, you are more likely to have a less severe case of the illness if you have had a flu shot.

What About Resistance to Antiviral Drugs?

Each year the flu appears to be increasingly resistant to antiviral medications. There are currently four antiviral medications approved by the FDA to help lessen symptoms or shorten the duration of the flu. However, two of these medications, amantadine and rimantadine, are almost completely ineffective against most strains of the flu, so the CDC does not recommend using these medications to treat it.

Tamiflu (oseltamivir) and Relenza (zanamivir) are more effective against the viruses, although increasing resistance among one type of influenza A virus has been noted with Tamiflu. At this time, the resistance is low enough that the CDC still recommends these medications for use when needed. However, only a small number of viruses have been tested, so these recommendations could change in the future.

What You Can Still Do to Prevent the Flu

If you haven’t been vaccinated against the flu, there are still steps that you can take to protect yourself from the flu. Because influenza is passed easily from one person to another, an important step to take to prevent any illness is to frequently wash your hands.

For many people, taking an antiviral medication, such as Tamiflu, within the first 48 hours of flu symptoms will help reduce the severity and length of the illness. Because these drugs are only available by prescription, you will have to see your healthcare provider. If you have been exposed to someone with the flu, an antiviral medication may be prescribed to help protect you from getting the virus.

A Word From Verywell

The CDC recommends these three easy steps to help protect you and your family from the flu this year:

  1. Get Vaccinated
    1. Although it is typically recommended that people get flu shots in the fall, they can still be effective if you get them in the spring. If the flu is arriving in your area, you may benefit from a flu shot. It takes about two weeks to get protection from a flu shot.
  2. Use Common Sense and Everyday Protection Steps
    1. Steps like washing your hands frequently and covering your mouth when you cough will protect you and others.
  3. Use Antiviral Medications
    1. If your doctor believes that antiviral medications will benefit you, using them may help protect you against the flu, reduce the severity of your symptoms, or the duration of your illness.

Take all of the steps you can to protect yourself and your family from the flu. It's not a mild or minor disease.

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Article Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial policy to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. CDC. Selecting viruses for the seasonal influenza vaccine. Updated September 4, 2018.

  2. CDC. How the flu virus can change: “drift” and “shift.” Updated October 15, 2019.

  3. University of Chicago Medicine. Why doesn't the flu vaccine work sometimes? Updated February 20, 2018.

  4. CDC. Antiviral drug resistance among influenza viruses. November 3, 2016.

  5. CDC. What you should know about flu antiviral drugs. Updated April 22, 2019.

  6. Cleveland Clinic. How to avoid catching the flu and other viral illnesses.

  7. Cleveland Clinic. 7 answers on Tamiflu: is it best to help you fight flu? Updated January 8, 2020.

  8. CDC. Take three actions to fight flu infographic. Updated September 16, 2019.

  9. CDC. Key facts about seasonal flu vaccine. Updated December 2, 2019.

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