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What's Herd Immunity?

People with fragile health are more protected when most of the human “herd” is vaccinated against flu, experts say.


Infographic on herd immunity from CSL Behring


Don’t be offended if someone asks you to be part of the herd during flu season.

“Herd immunity” is a way of thinking about immunizations and public health. The concept has for decades helped to protect people, especially children, from deadly illnesses. The Immune Deficiency Foundation, a patient advocacy organization for people who have immunodeficiency diseases, recently urged those who can to get a flu shot.

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“Herd immunity, where nearly everyone in the community is vaccinated, is the best way to protect those whose immune systems are fragile, like people with primary immunodeficiency diseases,” IDF President and CEO John G. Boyle said. “Getting a flu shot is an act of kindness.”

How does herd immunity work? When people are vaccinated and the vaccine prevents the flu, then two good things happen:

  1. Fewer people get the flu.

 2. Fewer people spread the flu.

It’s a numbers game, said Michael Blaese, MD, the IDF’s Consulting Medical Director.

“If you have 80 percent already immune to flu, then there are many fewer potential targets for the virus to find among the entire herd,” he said.

Every year, the flu is fatal for some patients, including children. The U.S. Centers for Disease Control reported that 85 percent of the children who died during the most recent flu season were not vaccinated. Some of the more serious complications of the flu include pneumonia and inflammation of the heart, according to the IDF flu guidelines.

“Please get a flu shot and protect not only yourself but also those in your community whose immune systems are at risk,” Boyle said.

Families and friends of those who have weaker immune systems are especially encouraged to join the herd. Someone who has a primary immunodeficiency disease, of which there are more than 350 types, has an immune system that is missing important parts or doesn’t work properly.

We want to create a ‘protective cocoon’ of immunized persons surrounding our patients so that they have less chance of being exposed,” the guidelines explain. And all the better if that cocoon can extend to schools and workplaces. 

The best time to get a flu vaccine is early in the season (October in the United States), according to the CDC. While the shot is still effective for those who get it later in the fall or winter, getting vaccinated before flu season is in full swing offers the best protection, the CDC says.