Figure 2.

The basic reproduction number and the characteristics of epidemics. (a) Illustration of an epidemic with discrete generations. If the basic reproduction number, R₀ is greater than 1 (here R₀=1.75) then the epidemic expands exponentially. The effective reproduction number, R, is calculated as the number of new infections divided by the number of infected individuals in the previous generation.¹⁵ (b) The characteristic shape of an uncontrolled outbreak where R₀>1. Initially the epidemic may die out because of stochastic factors, but once it is established it grows exponentially until susceptibles are exhausted at which point the epidemic slows until the disease either becomes endemic or extinct.¹⁶ (c) Estimating R₀ from the first month of cases in Hong Kong, shown by date of onset, as published by the WHO⁵ R₀ may be estimated from the exponential growth rate, here the slope of the log-linear fitted line, r=0.15. See text for estimation of R₀.