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

*P-value **P-value 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.