Influenza virus shedding and transmission in households

Abstract

Background The dynamics of influenza virus transmission are not yet fully understood, hindering the appropriate development and implementation of control and mitigation strategies. One major uncertainty relates to the profile of infectiousness over time in humans infected by influenza virus, and how variation in infectiousness might contribute to the risk of transmission in households.

Methods In 2008 and 2009, two large community-based studies were conducted to study the household transmission of influenza viruses in Hong Kong. I analyzed data on viral shedding and disease in participants, and used statistical models to examine how viral shedding patterns and other factors might affect the risk of influenza virus transmission in households, both within individuals over time, and between individuals with different patterns in viral shedding.

Results The patterns of viral shedding relative to the time of illness (acute respiratory illness; ARI) onset in naturally acquired infections were found to be largely comparable to the patterns observed in experimental infections. Viral shedding detected by RT-PCR peaks around the day of ARI onset after which levels of shedding declined over around 7 days, and viral shedding tended to be greater in children than adults. The patterns of viral shedding in cases of seasonal A subtypes were similar, although the trends of shedding in cases of seasonal B differed with some indication of a plateau in shedding for up to 5 days after illness onset. The risk of household influenza transmission was significantly associated with log10 viral shedding, though not with influenza related signs and symptoms such as cough.

Conclusions The patterns of viral shedding observed in naturally-acquired influenza A virus infections correlated with the pattern of infectiousness over time after onset of illness. The majority of infectiousness was estimated to occur within 2-3 days of illness onset, with implications for isolation strategies. The heterogeneous nature of individual viral shedding suggests the possibility of substantial variation in infectiousness, particularly among children.