Secondary infection risks by characteristics of household influenza index cases^a: the Household Influenza Vaccine Effectiveness (HIVE) study, Ann Arbor Michigan, 2010-2011 influenza season.

*P-value < 0.05 from Chi-square test for independence of outcome across levels of categorical exposure. **P-value < 0.001 from Chi-square test for independence of outcome across levels of categorical exposure. aOnly the first household introductions of influenza are considered here; 15 index/co-index cases, and 4 secondary cases were excluded. bReverse-transcriptase real-time polymerase chain reaction (RT-PCR) assay cycle-threshold (Ct) values are inversely related to viral load (i.e. lower Ct values are associated with higher viral load). cOne index case had an influenza A (pH1N1) and influenza B coinfection. Household contacts exposed to this index case are included in both the influenza A (pH1N1) and influenza B secondary infection risk estimates.