What is Herd Immunity?

Posted on February 3, 2017   by Anand Jagatia

When you think of a herd, you probably think of cows, wildebeest or buffalo. In the animal world, there is safety in numbers – more pairs of eyes to look out for predators, for example.

As humans, we don’t generally have to worry about predators, but we can gain the protection of the herd in other ways. “Herd immunity” is the idea that, as long as enough people in a population are immune to a disease (usually through vaccination), they can indirectly protect people who aren’t immune from getting infected.

Dr Adam Kucharski is an Assistant Professor of Epidemiology at the London School of Hygiene and Tropical Medicine, where he works on mathematical models of the spread and control of disease.

Read more about halting epidemics in the latest issue of Microbiology Today.

The numbers that Adam uses for vaccine threshold and herd immunity come from this formula:


(Ro – 1) / Ro

Where the proportion of people you need to vaccinate is related to how transmissible the disease is (Ro).

The more easily the disease spreads, the greater the proportion of the population you need to vaccinate to get herd immunity.