💉 Vaccine Immunity Threshold Calculator
Estimate what share of a population needs to be immune, and how much vaccination coverage that requires, to stop a disease from spreading.
What is herd immunity threshold
The herd immunity threshold is the share of a population that needs to be immune to an infectious disease before its spread reliably slows and dies out, because each infected person on average infects fewer than one other person.
The formulas used
The ideal threshold is calculated as 1 − (1 ÷ R0), where R0 is the basic reproduction number — the average number of people one infectious person would infect in a fully susceptible population. Because vaccines aren’t perfectly effective, the vaccination coverage actually needed is higher: the threshold divided by the vaccine’s efficacy. If a vaccine’s efficacy is too low to reach the threshold even at 100% coverage, the calculator flags this.
Note: real-world disease dynamics also depend on population mixing patterns, waning immunity, and virus variants, which this simplified model does not capture. This tool is for general and educational purposes only.