The threshold for population immunity varies depending on the disease and its basic reproduction number (R0). The R0 represents the average number of secondary infections produced by one infected individual in a fully susceptible population. Diseases with a higher R0 require a higher percentage of the population to be immune to achieve population immunity. For example, measles, with an R0 of 12-18, requires around 95% immunity, whereas influenza, with an R0 of 1.5-2, may require around 60-70%.
