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. 2019 Apr;24(15):1900104.
doi: 10.2807/1560-7917.ES.2019.24.15.1900104.

Children under 10 years of age were more affected by the 2018/19 influenza A(H1N1)pdm09 epidemic in Canada: ‎possible cohort effect following the 2009 influenza pandemic

Affiliations

Children under 10 years of age were more affected by the 2018/19 influenza A(H1N1)pdm09 epidemic in Canada: ‎possible cohort effect following the 2009 influenza pandemic

Danuta M Skowronski et al. Euro Surveill. 2019 Apr.

Abstract

IntroductionFindings from the community-based Canadian Sentinel Practitioner Surveillance Network (SPSN) suggest children were more affected by the 2018/19 influenza A(H1N1)pdm09 epidemic.AimTo compare the age distribution of A(H1N1)pdm09 cases in 2018/19 to prior seasonal influenza epidemics in Canada.MethodsThe age distribution of unvaccinated influenza A(H1N1)pdm09 cases and test-negative controls were compared across A(H1N1)pdm09-dominant epidemics in 2018/19, 2015/16 and 2013/14 and with the general population of SPSN provinces. Similar comparisons were undertaken for influenza A(H3N2)-dominant epidemics.ResultsIn 2018/19, more influenza A(H1N1)pdm09 cases were under 10 years old than controls (29% vs 16%; p < 0.001). In particular, children aged 5-9 years comprised 14% of cases, greater than their contribution to controls (4%) or the general population (5%) and at least twice their contribution in 2015/16 (7%; p < 0.001) or 2013/14 (5%; p < 0.001). Conversely, children aged 10-19 years (11% of the population) were under-represented among A(H1N1)pdm09 cases versus controls in 2018/19 (7% vs 12%; p < 0.001), 2015/16 (7% vs 13%; p < 0.001) and 2013/14 (9% vs 12%; p = 0.12).ConclusionChildren under 10 years old contributed more to outpatient A(H1N1)pdm09 medical visits in 2018/19 than prior seasonal epidemics in Canada. In 2018/19, all children under 10 years old were born after the 2009 A(H1N1)pdm09 pandemic and therefore lacked pandemic-induced immunity. In addition, more than half those born after 2009 now attend school (i.e. 5-9-year-olds), a socio-behavioural context that may enhance transmission and did not apply during prior A(H1N1)pdm09 epidemics.

Keywords: A(H1N1)pdm09; A(H3N2); Canada; ILI; age; air-borne infections; epidemic; epidemiology; influenza; influenza virus; influenza-like illness; laboratory; laboratory surveillance; pandemic; sentinel surveillance; viral infections.

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Conflict of interest statement

Conflict of interest: DMS is Principal Investigator on grants received from the Canadian Institutes of Health Research and the Public Health Agency of Canada in support of this work. GDS has received grants for investigator-initiated studies unrelated to influenza vaccine from Pfizer and provided paid expert testimony for the Ontario Nurses Association, the Quebec Ministry of Justice and GSK. JBG has received research grants from Pfizer Inc. to conduct microbiological surveillance of Streptococcus pneumoniae. MK has received research grants from Roche, Siemens and Hologic for unrelated studies. SJD is a content expert consultant to Johnson and Johnson (Janssen) Pharmaceuticals on a literature search for point-of-care testing for respiratory viruses. Other authors have no conflicts of interest to declare.

Figures

Figure 1
Figure 1
Percentage distribution of unvaccinated influenza A(H1N1)pdm09 cases and controls by single year of age, Canadian Sentinel Practitioner Surveillance Network (SPSN), seasonal influenza A(H1N1)pdm09 epidemics 2013/14, 2015/16, 2018/19
Figure 2
Figure 2
Percentage distribution of unvaccinated influenza A(H3N2) cases and controls by single year of age, Canadian Sentinel Practitioner Surveillance Network (SPSN), seasonal influenza A(H3N2) epidemics 2014/15, 2016/17, 2017/18
Figure 3
Figure 3
Percentage distribution of unvaccinated cases and controls, paediatric and elderly adult age groups, Canadian Sentinel Practitioner Surveillance Network (SPSN), seasonal influenza epidemics 2013/14–2018/19

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