How Herd Immunity Really Works

February 24th 2017

Kyle Jaeger

Though most of the U.S. population gets vaccinated for common diseases as children, the immunization rate for some conditions is still too low to achieve what's called "herd immunity."

Herd immunity is the scientific concept that if a certain percentage of the population is vaccinated, the entire population will be protected from contracting those diseases. That percentage varies depending on how contagious a given disease is, though.

Here's how many people need to be vaccinated to prevent the spread of seven diseases.


And here's the percentage of people currently immunized for those diseases, according to the Centers for Disease Control and Prevention (CDC).

  • Mumps: 91.5 percent
  • Polio: 93.3 percent
  • Diphtheria: 84.2 percent
  • Rubella: 91.5 percent
  • Pertussis: 84.2 percent
  • Measles: 91.5 percent

In theory, the U.S. population should be effectively immunized when it comes to mumps, polio, and rubella — meaning there should be virtually no cases that emerge. In practice, it's not that simple.

There were 5,311 cases of mumps reported to the CDC in 2016, following numerous outbreaks across the country. That shouldn't happen when more than 90 percent of the population is immunized, according to the basic principles of herd immunity. Therein lies a common misconception about immunization, Marcel Salathé, an assistant professor at Pennsylvania State University, wrote in The Washington Post.


"The assumption underlying the calculation for herd immunity is that people are mixing randomly, and that vaccination is distributed equally among the population," Salathé wrote. "But that is not true."

As it happens, rates of vaccination are "highly variable on the level of states, counties, and even schools," Salathé wrote, and research has indicated that anti-vaccination sentiments tend to "spread in communities, which may in turn lead to polarized communities with respect to vaccination."

In other words, people who might feel skeptical about vaccines shouldn't depend on the rest of the population to get immunized in order to protect themselves or their children, Salathé argued. Rather than aiming for 80 or 90 percent immunization, there is a "scientific argument for a public health policy that aims at 100 percent vaccination coverage."