What is ‘herd immunity’ and why are we trying to reach it?

A generic image of a needle and medication.

During the last few months, many health experts have been talking about reaching “herd immunity” with the distribution of the COVID-19 vaccine. Over the Christmas weekend, Dr. Anthony Fauci said he estimates the country needs to reach 80-85% with the vaccine to attain herd immunity.

Here’s what that means.

Herd immunity is a concept used in discussions about vaccination, and signifies the threshold needed for a population as a whole to be protected from a virus.

The World Health Organization clarifies that herd immunity, or population immunity, “is achieved by protecting people from a virus, not by exposing them to it.”

The Mayo Clinic also describes the situation following the 1918 Spanish Flu outbreak. They say people who survived the pandemic were later immune to infection of the H1N1 flu, also referred to as swine flu.

Health experts warn that relying on community infection to create herd immunity could be dangerous; there are questions about how long the antibodies created by infection protect against reinfection, and a high level of community infection could lead to overwhelmed medical facilities and a high number of deaths.

When the threshold of herd immunity is reached, it means the vast majority of a population is vaccinated and has antibodies, thereby lowering the overall amount of community spread of the virus. It also means portions of the population who cannot get vaccinated are kept safer by the limited spread.

What that threshold is varies depending on the virus and the efficacy of the vaccine.

According to the WHO, herd immunity against the measles is reached with 95% vaccination, while herd immunity against polio is 80%.

Dr. Fauci believes herd immunity for COVID-19 can be reached with around 85% of the population having antibodies.

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