What is herd immunity?

Herd immunity sometimes referred to as HIT for Herd Immunity Threshold, is the point at which a virus has more difficulty spreading because a significant amount of people have become immune. It is essential to know that once herd immunity is achieved, it does not mean a virus is gone, or everyone is immune at that point.

A simple way to think of it may be to think of a wildfire. When firefighters have stopped the fire from spreading, it is similar to having reached herd immunity in a pandemic. The fire is not out yet, and a danger of reignition exists until it is entirely extinguished. A change in the wind could expose the fire to a new location and start everything back up again.

The most basic formula for calculating herd immunity is 1-(1/R0), where R0 is the reproduction number. RO represents the number of people a person with the virus is likely to infect on average. It is not the number of people infected in the next twenty-four hours but the number of new cases per case. So, think of R0 as a measure of how infectious a virus is.

The larger the reproduction number is, the higher percentage of the population will need to be immune before herd immunity is reached. For example, a virus whose R0=2.1 makes the formula look like 1-(1/2.1)= .523, so 52.3% would need to be immune before herd immunity kicks in. Now a more infectious disease whose R0=12 would look like 1-(1/12)=.916, so 91.6% need to be immune. Here is a table of some well-known infectious diseases, their R0 number(s), and the necessary herd immunity percentage.

The R0 values of disease should be thought of as a scale for how infectious a disease is and not a precise value that never changes. For example, the R0 range of 12-18 that is quoted for measles was arrived at between 1912-1928 in the United States. If we were to lose herd immunity for measles it is hard to say what the R0 value would be today.

Since populations change over time, herd immunity also changes. All births, deaths, and migrations in or out of the community, have an affect on herd immunity. Anyone entering the population with immunity increases herd immunity as does anyone leaving the population without immunity. On the other hand anyone entering the population without immunity decreases herd immunity as does anyone leaving the population with immunity.

If you are interested in seeing a visual representation of a more advanced model, I’d suggest the model located at https://fivethirtyeight.com/features/without-a-vaccine-herd-immunity-wont-save-us/. It allows you to set three variables R0, death rate, and immunity duration then start a simulation that graphs for you how many days it takes for the HIT rate to be reached. No math required.