This should not be surprising, as the finding that a high proportion of influenza infections are asymptomatic or afebrile was not new. Published experimental data from volunteer studies had previously shown that 33% of proven seasonal influenza infections were asymptomatic, but this varied by influenza type and subtype [21] . In particular, as few as 37% of experimental infections with influenza A(H1N1) were associated with fever recorded as >37.8°C, while 30% were completely asymptomatic [21] . Moreover, viral shedding in the presymptomatic phase of influenza infection has recently been confirmed to occur in approximately 1-8% of naturally acquired infections, in a study in which 14% of all influenza infections were asymptomatic and 31% of infections with influenza A (H1N1 or H3N2) did not have fever at the onset of other symptoms [22] .