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  • Writer's pictureAndrea Alemanni

Why Did I Get the Flu if I Had the Vaccine?

Did you get the flu this season, even though you were vaccinated?

The CDC estimates 18-26 million people sought medical treatment due to flu-like symptoms from October 1, 2019 to April 4, 2020.

First, it’s important to note that getting an annual flu shot is strongly recommended—especially for children, the elderly, and those with compromised immune systems.

How the Flu Vaccine Works

Each year, scientists and health care professionals predict which flu strains are likely to be prevalent. The annual flu vaccine contains a small portion of these virus strains.

Your white blood cells are always ready to fight incoming disease. But they are like puzzle pieces, needing to fit onto the invading virus. If there’s an exact match, your chances of getting the flu are significantly reduced. If the white cells only partially fit or recognize the virus, you may still get sick. In this case, symptoms and length of illness will be much less compared with an unvaccinated person.

How the Flu Vaccine Makes a Difference

Your immune system is the key to protecting you from many diseases. When the flu virus enters the body of an unvaccinated person, the immune system produces antibodies to fight the invader. However, this person’s body has never “seen” the flu. It will take several days for their immune system to build enough white blood cells to fight Influenza. Meanwhile, the virus multiplies and invades healthy cells.

Contrast this with a vaccinated person who encounters the flu. Your immune system sees these foreign germs and builds white cells to fight it. Because the virus is weakened or inactivated, you develop antibodies to help protect you. Now your body can fight the invading virus with a ready arsenal. Your chances of contracting the “full-blown” flu are drastically reduced.

Facts About Influenza

· You can have the flu with NO fever.

· You can catch the flu October–May. Since 1982, the US peak month for flu activity is February.

· Shots are generally effective against the virus about two weeks after vaccination.

· There are several types of vaccines, including high-dose versions for people age 65+. Ask your health care provider about flu and other vaccine schedules.

· According to the CDC, the flu vaccine can reduce your risk of contracting the flu by 40–60%.

· The CDC DOESN’T consider antiviral drugs a substitute for the vaccine.

What About Other Vaccines?

Other scheduled childhood vaccinations and immunizations work the same way. By introducing a very small amount of the virus, the body builds immunity to the more dangerous version.

Modern vaccinations have nearly eliminated many dangerous and debilitating diseases, including polio, mumps, measles, and rubella. However, we are now seeing populations of children without vaccines experiencing “outbreaks” of both measles and chicken pox in the US.

Any risk in receiving vaccines are minimal and rare when compared to the effects of full exposure without protection.

Talk to your Health Care Professional about the best vaccine choices for you.


CDC (Centers for Disease Control)

Consumer Reports

NIH (National Institutes of Health)

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