It's hard to miss the ads on billboards, street signs, television, radio, and social media imploring us to get a flu shot. There's a good reason for those ads.  No other public health effort has had as profound an effect in controlling, and even eradicating, serious diseases.

However, it's also hard to miss the commonly expressed sentiment that getting a flu shot isn't necessary because "I don't get the flu" or "I'm healthy; why should I be vaccinated?" Whether or not you have ever had the flu, or had only a mild case, the vaccine is still important.

Why get a shot even if you don't think you'll get the flu? Because more is at stake than just your health. Your decision on whether to be vaccinated can affect the health of everyone around you. Public health professionals call this the herd effect. (This is very different from herd mentality, which does not make people more healthy.) It is a phenomenon that causes your vaccination status to affect others in your community.

If enough healthy individuals receive a vaccine against a disease, the germs that cause the disease can no longer persist in the community. Germs need a large base of susceptible individuals to survive. When the base becomes small enough, the entire "herd" is immune and those who are non-vaccinated receive the same protection as those who are vaccinated (assuming the vaccine is effective). Asking healthy people to be vaccinated may seem to be placing tremendous faith in their altruism, but they have a self-interest in getting a shot – they are protecting themselves, as well. It's a win-win situation.

Is herd immunity real? A large body of research emphatically says yes. For example, researchers at the University of Florida studied the incidence of the flu among schoolchildren during the 2011/2012 and 2012/2013 flu seasons. They found that in a community that offered opt-in vaccination for children aged 5 through 17 years, non-school age residents reported fewer cases of the flu than those in other communities that did not have the program. Thus, opt-in vaccination protected many individuals who did not, or could not for health reasons, receive the vaccine.

Public health officials strive to achieve the best vaccination rates possible, understanding that they will never reach 100 percent. But because of herd immunity, 100 percent compliance is not needed.

As more people decline vaccinations for non-medical reasons, the chance of achieving herd immunity shrinks, and the rest of the community faces a greater risk of becoming ill, and in some cases of dying, from a cause that is easily preventable. If we were to regard vaccination less as a burden like paying taxes, and more as a reward in the form of a chance for better health and a longer life, perhaps vaccination rates would rise and the coming flu season would be a mild one.

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Neal Goldstein, PhD, is an epidemiologist, writer, and public health advocate. He writes a science blog, which is available at www.goldsteinepi.com/blog, and can be followed on Twitter (@goldsteinepi).

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