Compared with case-patients who were not genotyped, the 105 case-patients with DNA available for genotyping were slightly older (median age 6 years vs. 4 years; p<0.05), less likely to have had a preexisting medical condition (28% vs. 61%; p<0.001), and less likely to have been vaccinated for infl uenza during the season of death (7% vs. 16%; p<0.01). Case-patients genotyped were more likely to have experienced sudden death (31% vs. 22%; p<0.05) and to have died before reaching medical care (34% vs. 22%; p<0.001). It is not surprising that case-patients with sudden death were more likely to have undergone autopsy and, hence, to have had tissues available for DNA extraction. Genotyped case-patients were less likely to have had pneumonia evident on chest radiograph (22% vs. 46%; p<0.05) and about equally likely to have had invasive bacterial co-infection (21% vs. 23%; not signifi cant), but differences in these characteristics are diffi cult to interpret because genotyped case-patients were less likely to have received medical care for their illnesses (presumably because of a greater frequency of sudden death).