"Under the simplest model, the herd immunity threshold depends on a single parameter known as R0, or the basic reproduction number (
Figure 2 A). R0 refers to the average number of secondary infections caused by a single infectious individual introduced into a completely susceptible population (
Anderson and May, 1985). If we consider a hypothetical pathogen with an R0 of 4, this means that, on average, one infected host will infect four others during the infectious period, assuming no immunity exists in the population. Mathematically, the herd immunity threshold is defined by 1 – 1/R0 (e.g., if R0 = 4, the corresponding herd immunity threshold is 0.75) (
Anderson and May, 1985). Therefore, the more communicable a pathogen, the greater its associated R0 and the greater the proportion of the population that must be immune to block sustained transmission (
Figure 2B). A similar parameter important for understanding population-level immunity is the effective reproduction number (Re or Rt). Re is defined as the average number of secondary cases generated by a single index case over an infectious period in a partially immune population (
Delamater et al., 2019). Unlike R0, Re does not assume a completely susceptible population and, consequently, will vary depending on a population’s current immune state, which will change dynamically as an outbreak event or vaccination campaign unfolds. Ultimately, the goal of vaccination programs is to bring the value of Re below 1. This occurs when the proportion of the population with immunity exceeds the herd immunity threshold. At this point, pathogen spread cannot be maintained, so there is a decline in the number of infected individuals within the population."