On the relative role of different age groups during influenza A epidemics in Germany, 2002-2017

TL;DR

This study analyzes the role of different age groups, especially school-age children, in propagating influenza A epidemics in Germany from 2002 to 2017, highlighting the importance of targeted vaccination strategies.

Contribution

It introduces a method to quantify the relative role of age groups in influenza spread using case data before and after epidemic peaks, applied to German data over 15 years.

Findings

School-age children, especially 14-17 years, often had the highest relative risk during epidemics.

Younger children and certain adult groups also played significant roles in specific seasons.

Vaccination coverage among older children is lower, potentially affecting epidemic dynamics.

Abstract

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.