What Is Herd Immunity??>
Ask the Experts

What Is Herd Immunity?

by Laurie Saloman  

Q: What is herd immunity?

A: Herd immunity occurs when enough individuals in a community have acquired immunity against a particular infection to make person-to-person transmission less likely. An individual’s immunity can be acquired by either getting vaccinated against the infection or contracting the infection. Herd immunity is a great example of safety in numbers.

We primarily need herd immunity to prevent or at least slow the spread of an infection under normal circumstances when social distancing or isolation isn’t being practiced. The greater the percentage of people in a community who are immune, the lower the chance that pathogens will be passed to uninfected people who aren’t immune. This protection is particularly important to individuals who can’t be safely vaccinated or whose immune systems won’t adequately respond to vaccination.

One challenge of herd immunity is that immunity to some infections can wane over time or because the virus changes. In some cases, people may need to be revaccinated regularly, as is done with annual flu shots. This points to another challenge, which is that herd immunity relies on the individual actions of citizens. A significant percentage of people don’t receive the recommended vaccinations, either because of cost, lack of access, or the belief that vaccines are harmful. When enough people living in proximity to each other don’t get vaccinated, it lowers the level of herd immunity and allows disease outbreaks, such as with clusters of measles in recent years.

This article first appeared in the July 2020 issue of UC Berkeley Health After 50.

Also see How to Fortify Your Immune System.