Having estimated the values ofRðBÞ andRðFÞ, equation (4.7) establishes a unique relationship between a and s(0). The reported clinical attack rate is the obvious first choice as an estimate of a, but may be biased for several possible reasons: (i) it is conceivable that clinical cases may not have conferred solid immunity, (ii) cases that seroconvert may be asymptomatic, and (iii) illness may have been mistakenly attributed to influenza when it was in fact caused by another influenza-like illness (e.g. respiratory syncytial virus). Hence, we explore the values of a to be an arbitrary 10% below (0.329) and 10% above (0.403) the reported clinical attack rate. The upper value turns out to be just above the maximum possible under our final approach to estimating a; that is, to use equation (4.7) under the assumption that everyone was initially susceptible to infection (i.e. s(0)Z1). We therefore explore three estimates of a. For each estimate, we compute the corresponding values of s(0), a max , a min , R A (B), R A (D), R A (F) and s min . We estimate R 0 as the average of R A (B), R A (D) and R A (F).