Fig. 1.

Estimated influenza illness and infection household secondary attack rates from this study and a PubMed literature search. Detailed information on the search references is given in (Sec. 2 (15) and Table S8). The household illness secondary attack rate is based on onset date of an influenza-like illness. Lab confirmed illness is confirmed through a virus-positive nasopharyngeal or throat swab taken at the time of the influenza-like illness. The household infection secondary attack rate is based on paired sera bracketing the usual influenza season, where an infection is defined as a significant rise in hemagglutination-inhibition titer comparing the pre-influenza season sample to the post-influenza season sample. The 95% confidence intervals are taken from the referenced paper or calculated by the authors if sufficient information was presented. Estimates from pandemic strains include the current estimate and those from Asian influenza A (H2N2) in 1957. The influenza A (H1N1) strain of 1978-1979 re-emerged after being absent since 1957. The influenza A (H5N1) strain in 2006 was an avian strain that did not spread beyond the initial family clusters.