Herd Immunity Threshold

When enough members of a population become immune to a contagious disease, that disease can stop spreading altogether, even among the small fraction of people who remain susceptible -- a protective effect known as herd immunity. The specific proportion of a population that needs to be immune before this protection takes hold is called the herd immunity threshold, and it isn't a fixed number that applies equally to every disease. Instead, it depends directly on how contagious a particular disease is, a property captured by a value epidemiologists call the basic reproduction number, or R0, which represents how many new people a single infected person would be expected to infect in a population with no immunity at all. A highly contagious disease with a high R0 requires a much larger proportion of the population to be immune before transmission chains reliably break down, while a less contagious disease with a lower R0 can be contained with comparatively fewer immune individuals. Once the proportion of immune individuals in a population exceeds the threshold calculated from a disease's R0, an infected person is statistically unlikely to encounter enough susceptible people to sustain an ongoing chain of transmission, and outbreaks tend to fizzle out on their own rather than growing into sustained epidemics. Crucially, herd immunity protects even the individuals who remain susceptible, including people who cannot be vaccinated for medical reasons, since the disease simply runs out of new hosts to infect before it can reach them. Estimating the herd immunity threshold accurately is complicated in practice, since a population's true level of immunity and a disease's real-world transmissibility can both shift over time as new variants emerge or immunity wanes.