Have you ever watched a wildlife show where bison or elephants are attacked by wolves or lions? The strong of the herd encircle the young and weaker members while keeping their horns or tusks pointed outward at their attackers. The wolves or lions are defeated because they cannot get to the weaker members of the herd.

This illustrates what health care providers mean when they talk about the importance of herd immunity in relationship to vaccines.

The good thing is that we don’t have to grow horns or tusks to protect our “herd.” We just need good immunity to protect from the likes of the flu, measles, mumps, chickenpox and a host of other diseases. There are two ways to develop immunity. The first is to have the actual illness. For example, when a person develops measles their immune system fights the virus, eliminates it and the person recovers. If that person encounters the measles virus again, the immune system remembers the virus and quickly wipes it out. The person has “immunity” against measles and remains well. The second, safer and more innocuous way to develop immunity is by being vaccinated against the disease.

Imagine a traveler with measles entering a community where a great majority of the citizens have immunity. The traveler is unlikely to meet someone who isn’t immune. The measles ends with the traveler, unable to spread to the susceptible members of the community. Even if the traveler meets a susceptible person and gives him the measles, that community member with measles is unlikely to run into other susceptible community members. Measles will fail to spread in this community.

We rely on herd immunity because vaccines aren’t perfect. Not everyone who’s vaccinated develops immunity. These individuals are referred to as non-responders. Additionally, there are members of our community that cannot be vaccinated because of medical conditions like cancer that affect their immune systems. Babies require repeated vaccination to develop immunity, which leaves them vulnerable until they have completed their vaccine series. Just about every other member of a community must be vaccinated to account for the number of susceptible among them.

The importance of herd immunity has been demonstrated time and again. Through vaccination, measles was eradicated from the United States by the year 2000. Over the years vaccination rates have fallen in pockets within communities created by groups of like-minded people who chose not to vaccinate. The pockets threaten everyone as the people in them don’t remain isolated — but venture out among us. They go to the same daycares, schools, grocery stores, medical facilities, amusement parks and ball games as everyone else. When there are enough of these pockets, spread of diseases like measles will be maintained in communities. Measles will not remain eradicated from the United States.

Do what’s right for you and for your “herd.” Stay up to date with your vaccinations and help maintain herd immunity. Remember, the wolves and lions are circling.

Vaccine Smarts is written by Sealy Institute for Vaccine Sciences faculty members Drs. Megan Berman, an associate professor of internal medicine, and Richard Rupp, a professor of pediatrics at the University of Texas Medical Branch. For questions about vaccines, email vaccine.smarts@utmb.edu.

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