Herd Immunity Threshold Calculator

Herd immunity occurs when a sufficient percentage of a population becomes immune to a disease, making its spread unlikely and protecting those who are not immune. It creates a community-wide shield against infectious disease outbreaks. See also our White Blood Cell Differential Calculator.

The basic formula is (1 - 1/R0) × 100%, where R0 is the basic reproduction number. For example, if R0 is 3, the threshold is (1 - 1/3) × 100% = 66.7%. This means about 67% of the population needs immunity.

R0 represents the average number of people that one infected person will infect in a completely susceptible population. Higher R0 values indicate more contagious diseases that require higher immunity thresholds.

Vaccine effectiveness affects how many people need to be vaccinated to achieve the required immunity level. If a vaccine is 95% effective, you need to vaccinate more people than the basic threshold to account for the 5% who won't develop immunity.

Yes, people who recover from infection typically develop immunity, which counts toward the herd immunity threshold. However, relying solely on natural infection can result in significant illness, death, and long-term health consequences.

Measles has an R0 of 12-18, COVID-19 variants range from 2-6, seasonal flu is about 1.3, and polio is around 5-7. Higher R0 values require higher vaccination coverage to achieve herd immunity.

Herd immunity only applies to diseases that spread person-to-person. It doesn't protect against diseases acquired from animals (like rabies) or environmental sources (like tetanus), where individual immunity is crucial.

Real-world populations don't mix uniformly. Age groups, geographic clusters, and social networks can create pockets of susceptibility where diseases can still spread even when overall immunity exceeds the theoretical threshold.