  Background: There is limited information about the role of different age
groups, particularly subgroups of school-age children and younger adults in
propagating influenza epidemics.
  Methods: For a communicable disease outbreak, some subpopulations may play a
disproportionate role during the ascent of the outbreak due to increased
susceptibility and/or contact rates. Such subpopulations can be identified by
considering the proportion that cases in a subpopulation represent among all
cases in the population occurring before the epidemic peak (Bp), the
corresponding proportion after the epidemic peak (Ap), to calculate the
relative risk for a subpopulation, RR=Bp/Ap. We estimated RR for several age
groups using data on reported influenza A cases in Germany between 2002-2017.
  Results: Children aged 14-17y had the highest RR estimates for 7 out of 15
influenza A epidemics in the data, including the 2009 pandemic, and the large
2016/17, 2008/09, and 2006/07 seasons. Children aged 10-13y had the highest RR
estimates during 3 epidemics, including the large 2014/15 and 2004/05 seasons.
Children aged 6-9y had the highest RR estimates during two epidemics, including
the large 2012/13 season. Children aged 2-5y had the highest RR estimate during
the moderate 2015/16 season; adults aged 18-24y had the highest RR estimate
during the small 2005/06 season; adults aged 25-34y had the highest RR estimate
during the large, 2002/03 season.
  Conclusions: Our results support the prominent role of all school-age
children, particularly the oldest ones, in propagating influenza epidemics in
the community. We note that national vaccination coverage levels among older
school-age children were lower than among younger school-age children during
the recent influenza seasons in the US, and influenza vaccination program in
England has not been phased in yet for secondary school students.
