At the
time, there was considerable uncertainty as to the pathogenic potential
of this virus but data soon showed the severity for humans to be less
than that seen with the 1918 pandemic but on a par with the 1957
‘‘Hong Kong’’ pandemic. Transmissibility appeared higher than is
normally the case for seasonal influenza with a higher than normal
attack rate. Importantly, younger age groups appeared more susceptible,
possibly due to partial immunity among older cohorts as a result
of being infected during previous pandemics.