Does influenza transmission occur from asymptomatic infection or prior to symptom onset?

A better understanding of transmission dynamics is essential in influenza pandemic planning. If a substantial proportion of transmissions were to occur during the presymptomatic phase or from asymptomatic individuals, then infection control measures such as contact tracing and quarantine of exposures would be of limited value. Infectiousness has been inferred based on the presence of influenza in the upper respiratory tract rather than from transmission experiments. Although asymptomatic individuals may shed influenza virus, studies have not determined if such people effectively transmit influenza. We performed a systematic review of published studies describing the relationship between viral shedding and disease transmission. Based on the available literature, we found that there is scant, if any, evidence that asymptomatic or presymptomatic individuals play an important role in influenza transmission. As such, recent articles concerning pandemic planning, some using transmission modeling, may have overestimated the effect of presymptomatic or asymptomatic influenza transmission. More definitive transmission studies are sorely needed.