The virulence of pandemic influenza A (H1N1) 2009: an epidemiological perspective on the case–fatality ratio

Abstract

When the pandemic influenza A (H1N1) 2009 virus emerged, one of the early priorities was to estimate its virulence. This is measured by the case–fatality ratio (CFR), the proportion of deaths from influenza among the total number of cases. Epidemiological studies in the very early stages of the pandemic estimated the confirmed CFR (cCFR), using confirmed cases as the denominator, to be approximately 0.5%. However, later studies estimated the symptomatic CFR (sCFR) as approximately 0.05% of all medically attended symptomatic cases. Although subjective, the virulence of influenza A (H1N1) 2009 can be perceived as mild. Further epidemiological investigations showed that both the cCFR and sCFR varied greatly by age and risk group. When assessing the efficacy of a specific intervention in reducing the risk of influenza-related death, particular care is required regarding the inclusion criteria and in matching age and underlying conditions between patients with, and without, the intervention.

Keywords::

• age-specific death rate
• case–fatality ratio
• comorbidity
• epidemiology
• influenza
• pandemics
• respiratory tract diseases
• survival analysis
• virulence

Financial & competing interests disclosure

Hiroshi Nishiura was supported by the PRESTO program, Japan Science and Technology Agency. The study sponsor had no role in the study design, data analysis, manuscript writing or the decision to submit for publication. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.