Community immunity, often referred to as herd immunity, refers to a situation in which enough of a population is immune to an infectious disease, either through vaccination or prior infection, to largely stop transmission of the disease and indirectly protect those who aren’t immune. The more contagious the disease, the higher proportion of people need to have immunity to get community immunity.
This classical definition applies to diseases such as measles, for example, which is extremely contagious, but is largely a disease of the past in the U.S., since the vast majority of Americans have been vaccinated or contracted the disease. Occasional outbreaks occur after travel-related exposures, usually among the unvaccinated or in areas with lower vaccination rates, but those remain localized.
COVID-19, however, is very different from measles because immunity against infection is short-lived and because the coronavirus mutates. For these reasons, scientists say it’s unlikely that there’s a particular community immunity threshold populations can reach that will snuff out outbreaks in the same way.
The general concept that a population as a whole can build up immunity to better control and limit the damage of the virus, however, still applies.
“The black and white view of herd immunity, meaning we will get to some level that this virus eventually just goes away and we never have to deal with it — that is never going to happen, at least not in the next decade,” Johns Hopkins University epidemiologist David Dowdy told us. But, he added, “the idea that we as a population can build up immunity to this virus, particularly immunity to severe cases, is very real and is absolutely happening.”
In this more nuanced view of community immunity, widespread vaccination remains an important goal.