Individual correlates of infectivity of influenza A virus infections in households

Abstract

BACKGROUND: Identifying individual correlates of infectivity of influenza virus is important for disease control and prevention. Viral shedding is used as a proxy measure of infectivity in many studies. However, the evidence for this is limited. METHOD: In a detailed study of influenza virus transmission within households in 2008-12, we recruited index cases with confirmed influenza infection from outpatient clinics, and followed up their household contacts for 7-10 days to identify secondary infections. We used individual-based hazard models to characterize the relationship between individual viral shedding and individual infectivity. RESULTS: We analyzed 386 households with 1147 household contacts. Index cases were separated into 3 groups according to their estimated level of viral shedding at symptom onset. We did not find a statistically significant association of virus shedding with transmission. Index cases in medium and higher viral shedding groups were estimated to have 21% (95% CI: -29%, 113%) and 44% (CI: -16%, 167%) higher infectivity, compared with those in the lower viral shedding group. CONCLUSION: Individual levels of viral shedding measured by RT-PCR in the nose and throat was not strongly correlated with individual infectivity in households. Our study was underpowered to confirm a possible weak to moderate association of viral shedding with infectivity. Other correlates of infectivity should be examined in future studies.