Influenza A Virus Shedding and Infectivity in Households

Background. Viral shedding is often considered to correlate with the infectivity of influenza, but the evidence for this is limited. Methods. In a detailed study of influenza virus transmission within households in 2008-2012, index case patients with confirmed influenza were identified in outpatient clinics, and we collected nose and throat swab specimens for testing by reverse-transcription polymerase chain reaction from all household members regardless of illness. We used individual-based hazard models to characterize the relationship between viral load (V) and infectivity. Results. Assuming that infectivity was proportional to viral load V gave the worst fit, because it strongly overestimated the proportion of transmission occurring at symptom onset. Alternative models assuming that infectivity was proportional to a various functions of V provided better fits, although they all overestimated the proportion of transmission occurring > 3 days after symptom onset. The best fitting model assumed that infectivity was proportion to V-gamma, with estimates of gamma = 0.136 and gamma = 0.156 for seasonal influenza A(H1N1) and A(H3N2) respectively. Conclusions. All the models we considered that used viral loads to approximate infectivity of a case imperfectly explained the timing of influenza secondary infections in households. Identification of more accurate correlates of infectivity will be important to inform control policies and disease modeling.