The threshold level is primarily determined by the basic reproduction number (R0), which represents the average number of secondary infections generated by one infected individual in a fully susceptible population. When R0 is greater than 1, the disease can spread through the population. The threshold level for herd immunity (H) can be calculated using the formula:
H = 1 - (1/R0)
For example, if R0 is 3, the threshold level is:
H = 1 - (1/3) = 0.67 or 67%
This means that 67% of the population must be immune to prevent the disease from spreading.
