• Open Access

Estimating age-specific cumulative incidence for the 2009 influenza pandemic: a meta-analysis of A(H1N1)pdm09 serological studies from 19 countries

Authors

  • Maria D. Van Kerkhove,

    1. Global Influenza Programme, WHO, Geneva, Switzerland
    2. MRC Centre for Outbreak Analysis and Modelling, Imperial College London, London, UK
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    • These authors contributed equally to this work.
  • Siddhivinayak Hirve,

    1. Global Influenza Programme, WHO, Geneva, Switzerland
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    • These authors contributed equally to this work.
  • Artemis Koukounari,

    1. MRC Centre for Outbreak Analysis and Modelling, Imperial College London, London, UK
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  • Anthony W. Mounts,

    Corresponding author
    • Global Influenza Programme, WHO, Geneva, Switzerland
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  • for the H1N1pdm serology working group

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    • Members of the H1N1pdm Serology Working Group Include: Regina Allwinn, Don Bandaranayake, Antonino Bella, Angie Bone, Fabrice Carrat, Mandeep S. Chadha, Mark Chen, Chia-Yu Chi, Chad M. Cox, Michelle Cretikos, Natasha Crowcroft, Jeffery Cutter, Xavier de Lamballerie, Koussay Dellagi, George Doukas, Sandra Dudareva-Vizule, Alicia M. Fry, Gwendolyn L. Gilbert, Walter Haas, Pia Hardelid, Peter Horby, Q. Sue Huang, Olav Hungnes, Niina Ikonen, Kyoko Iwatsuki-Horimoto, Naveed Z. Janjua, Iikka Julkunen, Jacqueline M. Katz, Yoshi Kawaoka, Ajit Lalvani, Daniel Levy-Bruhl, Helena C. Maltezou, Jodie McVernon, Elizabeth Miller, AC Mishra, Mohsen Moghadami, Shailesh D. Pawar, Carrie Reed, Steven Riley, Caterina Rizzo, Laura Rosella, Ted M. Ross, Yuelong Shu, Danuta M. Skowronski, Saranya Sridhar, Anneke Steens, Babasaheb V. Tandale, Maria Theodoridou, Michiel van Boven, Kristian Waalen, Jen-Ren Wang, Joseph T. Wu, Cuiling Xu, Shanta Zimmer, Christl A. Donnelly, and Neil M. Ferguson.
    • Affiliations details are mentioned in the Appendix part.

  • The opinions expressed in this article are those of the authors and members of the working group and do not necessarily reflect those of the institutions or organizations with which they are affiliated.

Correspondence: Anthony W. Mounts, Global Influenza Programme, World Health Organization, Geneva, Switzerland. E-mail: mountsa@who.int

Abstract

Background

The global impact of the 2009 influenza A(H1N1) pandemic (H1N1pdm) is not well understood.

Objectives

We estimate overall and age-specific prevalence of cross-reactive antibodies to H1N1pdm virus and rates of H1N1pdm infection during the first year of the pandemic using data from published and unpublished H1N1pdm seroepidemiological studies.

Methods

Primary aggregate H1N1pdm serologic data from each study were stratified in standardized age groups and evaluated based on when sera were collected in relation to national or subnational peak H1N1pdm activity. Seropositivity was assessed using well-described and standardized hemagglutination inhibition (HI titers ≥32 or ≥40) and microneutralization (MN ≥ 40) laboratory assays. The prevalence of cross-reactive antibodies to the H1N1pdm virus was estimated for studies using sera collected prior to the start of the pandemic (between 2004 and April 2009); H1N1pdm cumulative incidence was estimated for studies in which collected both pre- and post-pandemic sera; and H1N1pdm seropositivity was calculated from studies with post-pandemic sera only (collected between December 2009–June 2010).

Results

Data from 27 published/unpublished studies from 19 countries/administrative regions – Australia, Canada, China, Finland, France, Germany, Hong Kong SAR, India, Iran, Italy, Japan, Netherlands, New Zealand, Norway, Reunion Island, Singapore, United Kingdom, United States, and Vietnam – were eligible for inclusion. The overall age-standardized pre-pandemic prevalence of cross-reactive antibodies was 5% (95%CI 3–7%) and varied significantly by age with the highest rates among persons ≥65 years old (14% 95%CI 8–24%). Overall age-standardized H1N1pdm cumulative incidence was 24% (95%CI 20–27%) and varied significantly by age with the highest in children 5–19 (47% 95%CI 39–55%) and 0–4 years old (36% 95%CI 30–43%).

Conclusions

Our results offer unique insight into the global impact of the H1N1 pandemic and highlight the need for standardization of seroepidemiological studies and for their inclusion in pre-pandemic preparedness plans. Our results taken together with recent global pandemic respiratory-associated mortality estimates suggest that the case fatality ratio of the pandemic virus was approximately 0·02%.

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