How does herd immunity work?

Herd immunity occurs when enough people become immune to a disease that the virus has trouble transmitting to new hosts and can eventually stop. The two main ways to reach herd immunity are through natural immunity — when you get the virus and your body develops antibodies that protect it against future infection — or through vaccinations.

Depending on the virus, there is a certain percentage of the population that needs to achieve immunity before herd immunity can be reached — called the “herd immunity threshold.”

For example, measles — an especially contagious disease — slows down only after about 95 percent of people become immune.

It remains unclear how large a portion of the population must become infected with the new coronavirus to reach that threshold. Estimates have ranged from 20 percent to 80 percent. As scientists have learned more about the virus, some have narrowed their estimates to between 40 and 70 percent.

Given the coronavirus’ transmissibility, it is likely about 65 to 70 percent of the population would need to become infected for there to be herd immunity.

Exactly how many people would die before a population can reach immunity depends on a number of complicated variables, but even rudimentary calculations show it is likely to be substantial.

With a population of 328 million in the United States, it may require more than 2 million deaths to reach a 65 percent threshold of herd immunity, assuming the virus has a 1 percent fatality rate, according to an analysis by the Washington Post. Even if both the herd immunity threshold and the fatality rate proved to be toward the lower end of current estimates — with 40 percent needing to be infected and a 0.5 percent fatality rate — the country could still expect 656,000 deaths to achieve herd immunity.