Elsevier

Journal of Infection

Volume 63, Issue 4, October 2011, Pages 243-251
Journal of Infection

Review
Factors associated with severe illness in pandemic 2009 influenza a (H1N1) infection: Implications for triage in primary and secondary care

https://doi.org/10.1016/j.jinf.2011.07.014Get rights and content

Summary

Pandemic (H1N1) 2009 influenza virus (pH1N1/09) infection spread rapidly around the globe, leading to a phase 6 pandemic level of alert declared in June 2009. The WHO declared the end of the pandemic in August 2010. Although for the majority of infected patients, it manifest as a mild, self-limiting illness, a proportion appeared to follow an adverse clinical course, requiring higher level care and aggressive management strategies. Experience with previous pandemics suggests that H1N1 will continue to circulate for many years. The aim of this review is to evaluate data from published case series reporting patients with pH1N1/09 influenza to identify clinical markers of severe disease. Comorbid illnesses including chronic lung disease, obesity and pregnancy have been shown to confer increased risk of severe infection. Admission vital signs, laboratory investigations and chest radiographic features can guide admitting clinicians to stratify patients’ risk of severe disease, however, the currently available severity scoring tools have only a limited role in risk assessment. Knowledge of high risk parameters remains important for clinicians triaging patients with suspected pH1N1/09 influenza and to inform strategies for future pandemics.

Introduction

Pandemic H1N1 2009 influenza virus (pH1N1/09) infection spread rapidly around the globe following its first detection in April 2009, leading to a phase 6 global pandemic level of alert declared in June 2009.1 Over 600,000 cases of laboratory confirmed pH1N1/09 were reported worldwide during the pandemic, however the numbers treated and not reported are likely to be considerably higher.2 While a large proportion of cases were mild, there were a total of 14,286 reported deaths3 with a case-fatality rate estimated between 0.1% and 0.7%.4 9–31% of hospitalised patients required admission to the ICU.5, 6 Importantly, this included a significant number of deaths reported in previously healthy young adults.6

In winter 2010/2011, pH1N1/09 remained a dominant circulating influenza strain in the UK and was shown to be virologically and epidemiologically similar to the 2009 pandemic strain.7 To ensure preparedness for potential future waves of pH1N1/09, identifying correlates for severity of disease remains important.

Evidence suggests that physicians both under and overestimate the severity of acute respiratory infections when relying on clinical judgement alone.8 Consequently, national guidelines for the management of influenza have recommended the use of severity and hospital admission criteria as an adjunct to clinical judgement.9, 10 The aim of this review is to evaluate published data on severe pH1N1/09 infection (with severe infection defined as mortality or requirement for ICU admission). By identifying clinical markers of severe disease and examining the national and international guideline recommendations for triaging pH1N1/09, we aim to provide clinicians with an evidence-based approach to triaging patients with pH1N1/09 and review the lesions learnt from the 2009/10 pandemic.

Section snippets

Methods and search strategy

The current review was based on a search of pubmed and embase articles published from 01 January 2008 to 01 April 2011 using search terms “H1N1”, “influenza”, “pandemic” and “swine”. We applied no language restriction. The inclusion criteria were 1) inclusion of patients with laboratory confirmed (PCR/viral culture/antibody testing) pH1N1/09 influenza virus infection OR probable (influenza A detected on PCR that is nonsubtypable for human subtypes H1 or H3) pH1N1/09 infection; 2) reporting data

Factors reported to be associated with severity in existing case series

Individual patient characteristics can be used to identify patients at risk of poor outcome in respiratory tract infections.8 The following sections describe the clinical, laboratory, radiographic and demographic factors reported to be associated with severe illness following pH1N1/09 infection.

Implications for severity assessment and relation to existing severity scores

Existing case series highlight a number of clinical and co-morbid variables associated with severe pH1N1/09 infection. However, whether the presence or absence of these factors on presentation can be useful for admitting clinicians to identify patients at increased risk of adverse outcome is unclear.

Severity scores currently available to aid triage decisions include: a) generic scores designed for use in sepsis/ICU admitted patients; b) pandemic influenza guidelines, not specific to H1N160 c) a

Conflict of interest

All authors declared no conflict of interest.

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