In 1923, University of Manchester bacteriologists W.W.C. Topley and G.S. Wilson coined the phrase herd immunity in a study of how bacterial infections spread through a group of mice when some had been inoculated [PDF]. Though they stressed that further research was needed on the subject, the basic idea was that a community might be able to achieve overall protection from an infectious disease if a certain proportion of its individual members were immune.
If, for example, a sick person came in contact with five people—four of whom were immune to the disease—that person could only pass it on to the one susceptible individual. Not only would the other four people remain healthy, but they also wouldn’t transmit the disease to whomever they encountered. This way, singular cases are much easier to contain, and a highly infectious disease becomes a much lower threat to an entire community.
Nearly a century after Topley and Wilson introduced the term, humans have seen herd immunity play out in real life more than a few times. As the Association for Professionals in Infection Control and Epidemiology explains, it often occurs when the majority of a population gets vaccinated against a certain disease. It’s still possible to contract chicken pox or measles, for instance, but so many people in the U.S. have received those vaccines that individual cases don’t easily become outbreaks (or pandemics).
Herd immunity can also happen when many individuals have already contracted a disease and now possess antibodies that make them immune to catching it again. Since we don’t have a vaccine for COVID-19 yet, some people are hoping we can achieve herd immunity this way. But according to Gypsyamber D’Souza and David Dowdy, epidemiology professors at the Johns Hopkins Bloomberg School of Public Health, recovered COVID-19 patients might only be immune to the disease “for months to years.” We don’t currently have enough information to know how reinfection works with this particular coronavirus, but we do know people can be reinfected with other coronaviruses. In other words, we shouldn't count on achieving herd immunity in this manner.
Furthermore, it’s estimated that at least 70 percent of individuals would need immunity to COVID-19 in order for the whole population to have herd immunity. Our healthcare system would quickly become overwhelmed with the number of cases and catastrophic number of deaths that would occur if people intentionally became infected to achieve immunity.
To summarize: Our best bet for conquering COVID-19 is to social-distance, wear a mask, and wait patiently for a vaccine.