Estimated secondary attack rates.

The secondary attack rates are defined as the person-to-person transmission probabilities over the complete infectious period of the infected person. Household members are categorized as younger (≤12 years of age, ‘y’) and older (>12 years of age, ‘o’). In model A the secondary attack rate does not depend on age, while in models B and C the secondary attack rates depend on the age of the infector (model B) or infected (model C). In models D and E the secondary attack rates depend both on the age of the infector and the age of the infected. In model E a separate transmission parameter is estimated for each transmission route, while in model D the secondary attack rates are based on the estimated relative susceptibility and infectiousness of older relative to younger persons. Calculation of the empirical support relative to the most likely model is based on the small sample Akaike's Information Criterion (AICc).