Herd immunity threshold depends on the basic reproduction number (R_0) of the disease:

H = 1 - \\frac{1}{R_0}

For measles, R_0 is 12–18 (highly contagious), making the herd immunity threshold approximately 92–95%. This is why measles can re-emerge even with 90% vaccination coverage. A Head Nurse must understand herd immunity concepts to advocate for achieving high vaccination coverage targets in EPI campaigns.