Herd Immunity: Understanding COVID-19

Abstract

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its associated disease, COVID-19, has demonstrated the devastating impact of a novel, infectious pathogen on a susceptible population. Here, we explain the basic concepts of herd immunity and discuss its implications in the context of COVID-19.

Figure 1

Herd Immunity (A) SIR (susceptible, infectious, recovered) model for a completely immunizing infection with an R₀ = 4. The model assumes a closed population in which no people leave and no new cases are introduced. Following the introduction of a single infected individual, the proportion of infected individuals (red line) increases rapidly until reaching its peak, which corresponds to the herd immunity threshold. After this point, newly infected individuals infect fewer than one susceptible individual, as a sufficient proportion of the population has become resistant, preventing further spread of the pathogen (orange line). (B) Schematic depiction of the disease propagation dynamics when one infected individual is introduced into a completely susceptible population (top panel) versus a situation in which an infected individual is introduced into a population that has reached the herd immunity threshold (bottom panel). In the naive population, an outbreak quickly emerges, whereas under the scenario of herd immunity, the virus fails to spread and persist in the population.

Figure 2

The Potential Health Burden of COVID-19 if Herd Immunity Is Achieved in the Absence of Vaccination (A) Relationship between R₀—the basic reproduction number (Box 1)—and the herd immunity threshold, which corresponds to the proportion of individuals in the population that would need be become immune for herd immunity to be established (y axis). As R₀ increases, the proportion of the population that must be immune to generate herd immunity increases (1 – 1/R₀). (B) Basic reproduction numbers (R₀) and the corresponding herd immunity thresholds for various infectious diseases. R₀ estimates represent the commonly accepted R₀ range for each of the pathogens reported. (C) Expected number of absolute deaths for the top 20 countries with the highest incidence of COVID-19 as of April 10, 2020, assuming herd immunity is established at a uniform threshold of 67% (R₀ = 3) in each country. Overall COVID-19 infection fatality rates (IFR) of 0.2%, 0.6%, and 1.0% are considered. We note that these numbers are necessarily underestimates given that, even after the herd immunity threshold is reached, it will take a long time until there are no more new cases, and therefore, no new deaths.