Changes in the age distribution of confirmed influenza cases in Alberta, Canada

DaiHai He; Jonathan Dushoff; Alice P. Y. Chiu; Mark B. Loeb; Kevin Fonseca; David J. D. Earn; DaiHai He; Jonathan Dushoff; Alice P. Y. Chiu; Mark B. Loeb; Kevin Fonseca; David J. D. Earn

doi:10.3934/bdia.2026003

Big Data and Information Analytics

2026, Volume 10, 53-69. doi: 10.3934/bdia.2026003

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Research article

Changes in the age distribution of confirmed influenza cases in Alberta, Canada

1.
Department of Applied Mathematics, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
2.
Department of Biology, McMaster University, Ontario, Hamilton, Canada
3.
M. G. DeGroote Institute for Infectious Disease Research, McMaster University, Ontario, Hamilton, Canada
4.
Department of Clinical Epidemiology and Biostatistics, McMaster University, Ontario, Hamilton, Canada
5.
Department of Microbiology & Infectious Diseases, University of Calgary, Alberta, Canada
6.
Department of Mathematics and Statistics, McMaster University, Ontario, Hamilton, Canada

Received: 17 December 2025 Accepted: 29 December 2025 Published: 21 January 2026

The age-incidence pattern during an influenza pandemic tends to be characterized by a relatively high proportion of severe cases in young adults. After a pandemic, the age pattern of those infected with the pandemic strain shifts toward that of seasonal strains, likely due to population immunity, and possibly viral evolution as well. We study age-incidence patterns of three influenza subtypes in Alberta, Canada from January 2009 to March 2015, a period that includes the 2009 pandemic. We find a consistent age drift toward older populations from A/H1N1pdm (pandemic strain) cases. Unexpectedly, we also find similar shifts toward the elderly for confirmed cases of seasonal influenza A/H3N2 and influenza B. The school term schedule also has clear effects on the age structure of reported cases. At the national level, we show the weekly reports for influenza from 2009 to 2025 and the season vs. age group vs. subtype patterns from 2015 to 2024. We find that A/H3N2 was the first subtype to return in Canada after the COVID-19 pandemic, followed by A/H1N1. We also highlight that A/H1N1 severely impacted the age 65+ group in the 2023–24 season.
- influenza,
- age group,
- 2009 influenza pandemic,
- COVID-19 pandemic
Citation: DaiHai He, Jonathan Dushoff, Alice P. Y. Chiu, Mark B. Loeb, Kevin Fonseca, David J. D. Earn. Changes in the age distribution of confirmed influenza cases in Alberta, Canada[J]. Big Data and Information Analytics, 2026, 10: 53-69. doi: 10.3934/bdia.2026003

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Abstract

The age-incidence pattern during an influenza pandemic tends to be characterized by a relatively high proportion of severe cases in young adults. After a pandemic, the age pattern of those infected with the pandemic strain shifts toward that of seasonal strains, likely due to population immunity, and possibly viral evolution as well. We study age-incidence patterns of three influenza subtypes in Alberta, Canada from January 2009 to March 2015, a period that includes the 2009 pandemic. We find a consistent age drift toward older populations from A/H1N1pdm (pandemic strain) cases. Unexpectedly, we also find similar shifts toward the elderly for confirmed cases of seasonal influenza A/H3N2 and influenza B. The school term schedule also has clear effects on the age structure of reported cases. At the national level, we show the weekly reports for influenza from 2009 to 2025 and the season vs. age group vs. subtype patterns from 2015 to 2024. We find that A/H3N2 was the first subtype to return in Canada after the COVID-19 pandemic, followed by A/H1N1. We also highlight that A/H1N1 severely impacted the age 65+ group in the 2023–24 season.

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