Reproductive number (R₀) of SARS-CoV-2 (original Wuhan strain) was approximately 2–3, while the Omicron variant had R₀ approximately 8–15. R₀ represents:

• A The case fatality rate in a fully susceptible population
• B The proportion of the population that must be immune to achieve herd immunity
• C The serial interval between successive cases in an epidemic chain
• D The average number of secondary infections generated by one infectious case in a fully susceptible population ✓

Explanation

Basic Reproduction Number (R₀) is the average number of secondary cases produced by one primary case in a totally susceptible population with no interventions. If R₀ >1, epidemic spreads; if <1, it dies out. The herd immunity threshold = 1 − 1/R₀ (so for R₀=15, HIT = 93.3%). Serial interval is the time between symptom onset of primary and secondary cases, a separate concept.

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

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