Timeline of our study rounds and community influenza virus activity.

<p>In total we collected blood in seven rounds, with each of the four numbered H3N2 epidemics being neatly bracketed by two consecutive rounds of blood draws. The y-axis shows weekly influenza virus activity in Hong Kong from 2009 to 2014, measured for each influenza type/subtype as the weekly proportion of outpatient consultations associated with influenza-like-illness in sentinel outpatient clinics multiplied by the weekly proportions of laboratory specimens testing positive for influenza A(H3N2), A(H1N1)pdm09 and B viruses respectively. For each type/subtype the activity level should correlate with incidence of infections within an epidemic, but changes in consultation behaviors between epidemics (e.g. in 2009/10) may also influence observed ‘activity’ levels.