Britannica Sources used in this answer

Why does measles require a higher level of immunity for herd immunity than tuberculosis?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Infographic showing estimated lives saved by childhood vaccinations worldwide from 1974 to 2024, with syringe-shaped bars for each disease. Measles tops the list at 93.71 million lives saved, followed by tetanus, whooping cough, and others, for a total of 153.84 million lives saved. — vaccine Vaccines and the estimates of lives saved over the period 1974–2024.

The requirement for a higher level of herd immunity for measles compared to tuberculosis is primarily due to the difference in their contagiousness, which is measured by the basic reproduction number, or $R_{0}$ .12

The $R_{0}$ represents the average number of secondary infections produced by a single infected individual in a fully susceptible population. Because measles is significantly more contagious, with an $R_{0}$ ranging from 12 to 18, it requires a much higher proportion of the population to be immune to prevent sustained spread. In contrast, tuberculosis has a lower $R_{0}$ , typically ranging from 1 to 4, meaning it is less efficient at spreading through a population and therefore requires a lower threshold of immunity to achieve herd immunity.12

The threshold for herd immunity ( $H$ ) is calculated using the formula $H > 1 - 1 / R_{0}$ . As $R_{0}$ increases, the percentage of the population that must be immune to break the chain of transmission also increases, making high vaccination coverage—often 95 percent or more for measles—essential for public health.12