Relative illness ratio (RIR) across age groups. The RIR is the ratio of the percentage of sick persons in a given age group to the percentage of the general population belonging to the same age group. The age distribution of morbidity were established from data based on cohorts of 6,000 patients during the 1978-79 seasonal epidemic and 59,785 patients during the 1989-90 seasonal epidemic in the US. The age distribution of morbidity were obtained from the report of 18,839 (1988-89) and 27,054 (1989-90) cases in France. Pandemic age-distribution were obtained from 11,485 (France) and 658,078 (US) cases.