The 'basic reproduction number' (R₀) of a pathogen represents the average number of secondary cases generated by one infectious case in a fully susceptible population. If the herd immunity threshold (HIT) for measles is approximately 95%, the corresponding R₀ value is:

• A 5
• B 10
• C 50
• D 20 ✓

Explanation

Herd immunity threshold (HIT) = 1 − (1/R₀). If HIT = 0.95, then 0.95 = 1 − (1/R₀), so 1/R₀ = 0.05, giving R₀ = 1/0.05 = 20. Measles has one of the highest R₀ values of any infectious disease (~12–20), explaining why it requires ~95% population immunity to interrupt transmission. A pathogen with R₀ = 5 would require only 80% herd immunity (1 − 1/5 = 0.80). This relationship is fundamental to understanding vaccination coverage targets for elimination.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.