Question: I hear a lot about herd immunity, but I’m not sure that I get it. Would you please explain?

Answer: According to this history of herd immunity published in 2011, the term “herd immunity” was coined by scientists back in the 1920s. The basic idea is that if enough individuals in a given population develop immunity to a virus (generally through immunization, sometimes by widespread infection) then the vulnerable individuals in that population will be protected too (Figure 1). Imagine a herd of water buffalo surrounding their calves to protect them from lions and now you’re imagining immunized individuals surrounding vulnerable individuals and protecting them from bad viruses (here’s the video — it has a happy ending)! I think you probably already had this part of the knowledge down, so let me get more into the weeds with it for a minute.

Generally, 70% to 90% of a population needs immunity to achieve herd immunity. The proportion required to achieve herd immunity varies based on four factors: 1) how contagious an infection is; 2) how much immunity is conferred by the vaccine (or previous illness); 3) the social networks within the population (e.g. how people mingle); 4) the distribution of the vaccine/immunity within the population (e.g. heterogeneity of the population vaccinated/infected). Based on current estimates of how contagious the coronavirus is, scientists estimate that at least 70% of the population needs to become immune in order for the remaining 30% to be protected.

If the United States were to try to achieve herd immunity before a vaccine comes to the fore, it would mean that 231 million of us would need to be infected. And this presupposes that the antibodies we develop as a result of infection will protect us from reinfection, which is still an area of scientific inquiry (see Q&A of 4/25). Data from serology testing in one big hot spot, New York City, suggest that ~21% of NYC residents have been infected. Despite the massively heavy toll the virus has taken on the City and her residents, NYC is still far from reaching herd immunity. Epidemiologists of Johns Hopkins recently discussed this very issue and made the compelling case in their very brief article that, “As infectious disease epidemiologists, we wish to state clearly that herd immunity against COVID-19 will not be achieved at a population level in 2020, barring a public health catastrophe.”

Figure 1. Virus transmission in Susceptible vs. Immune Populations (from this Vaccines paper)