Nosocomial vs community-acquired pandemic influenza A (H1N1) 2009: a nested case–control study

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Summary

Background

The characteristics of nosocomial influenza in children are not well described.

Aim

To compare the characteristics of nosocomial and community-acquired pandemic influenza A (H1N1) 2009 (pH1N1) in Australian children.

Methods

In a nested case–control study, the clinical and epidemiological features of nosocomial vs community-acquired pH1N1 were compared among hospitalized children aged <15 years in six paediatric hospitals in Australia between 1 June and 30 September 2009.

Findings

Of 506 hospitalized children with pH1N1, 47 (9.3%) were of nosocomial origin. These 47 cases were compared with 141 gender- and age-matched controls. Cases had a significantly higher proportion of underlying medical conditions compared with controls (81% vs 42%, P < 0.001), and were more likely to be exposed to household smokers (36% vs 20%, P = 0.02). Fewer children with nosocomial influenza presented with classical symptoms of influenza, including subjective fever and lethargy. A higher proportion of children with nosocomial influenza received treatment with oseltamivir (77% vs 43%, P < 0.001), and they required a longer stay in hospital following the onset of influenza (mean 8.5 days vs 4.5 days, P = 0.006). Three children (2%) in the community-acquired group died of pH1N1, but there were no deaths in the nosocomial group.

Conclusion

This study shows that children with pre-existing diseases and those who are exposed to household smokers are more susceptible to nosocomial pH1N1. They may have ‘occult presentation’ of influenza, but their course of illness is not markedly different from that of children with community-acquired influenza.

Introduction

Nosocomial or healthcare-associated influenza is documented in the medical literature,1, 2, 3, 4, 5, 6, 7 but to the authors' knowledge, there are no studies reporting comparative data on clinical and epidemiological features of nosocomial vs community-acquired influenza. Paediatric nosocomial influenza leads to extended hospital stay, excess morbidity and mortality, and economic loss.1 The latter has been estimated conservatively to be approximately US $7500 per patient for each additional stay in hospital.8 In epidemic/pandemic situations, this can strain an already overburdened healthcare system.

During the first wave of the pandemic influenza A (H1N1) 2009 (pH1N1), the rate of all-cause nosocomial infections doubled in some countries (e.g. Mexico) compared with previous years.9 Subsequently, there have been several case reports, case series and outbreak investigations on nosocomial pandemic influenza in children.10, 11, 12, 13 A recent surveillance report from the UK described 15 cases of paediatric nosocomial influenza; to the authors' knowledge, this is the largest published report of nosocomial pH1N1 to date.14

The literature suggests a variable presentation and outcome of hospital-acquired pH1N1 influenza. Most case series or outbreak investigations reported that paediatric nosocomial pH1N1 was generally mild, rarely needed aggressive medical care and had very low mortality.10, 11, 12, 13 However, the recent UK surveillance report indicated that the course and outcome of hospital-acquired pH1N1 can be severe both in terms of mortality and the need for escalated medical care.14 The report suggested that more than half of the children needed intensive medical care, and one-fifth of all children with nosocomial pH1N1 died. In order to compare the clinical and epidemiological features, risk factors and outcomes of nosocomial and community-acquired pH1N1 in children, a nested case–control study was conducted at six major paediatric hospitals in Australia. A nested case–control design was chosen for this study in order to achieve a statistically efficient analysis with matching for potential confounders.

Section snippets

Setting

This study is part of an active hospital-based surveillance system called ‘PAEDS’ (Paediatric Active Enhanced Disease Surveillance), which is a collaborative project between the Australian Paediatric Surveillance Unit (APSU) and the National Centre for Immunisation Research and Surveillance (NCIRS) that involves six tertiary paediatric referral centres in Australia (Children's Hospital at Westmead in Western Sydney, Sydney Children's Hospital in Eastern Sydney, John Hunter Children's Hospital

Demographics

There were 506 children with laboratory-confirmed pH1N1 in the PAEDS database, and 47 (9.3%) were cases of nosocomial pH1N1 influenza. Of the 459 (90.7%) children with community-acquired pH1N1 influenza, 141 (three per case) were selected as controls for the nested case–control study. The distribution of cases and controls across the hospitals ranged from two to 22 and from nine to 42 subjects per hospital, respectively. The demographic characteristics of the two groups are shown in Table I.

Risk factors

Discussion

This study has described the largest known cohort of children (N = 47) with nosocomial influenza in a single season, and the first nested case–control study on nosocomial vs community-acquired pH1N1. The only other large cohort of hospital-acquired influenza has been described from Philadelphia Children's Hospital, involving 46 children aged ≤21 years hospitalized over four influenza seasons from 2000 onwards.6

This study found that nosocomial influenza differed significantly from

Conflict of interest statement

RB has received financial support from CSL, Sanofi, GlaxoSmithKline, Novartis, Roche and Wyeth to conduct research and attend and present at scientific meetings. Any funding received is directed to an NCIRS research account at The Children's Hospital at Westmead and is not personally accepted by RB. HM has participated as a board member of global advisory boards for Merck and GlaxoSmithKline, and has participated as an investigator in industry-sponsored clinical vaccine trials. JB has served on

Funding sources

This project was funded, in part, by the New South Wales Department of Health and the National Health and Medical Research Council (NHMRC) H1N1 Grant No: 633028. Activities of the APSU and NCIRS are supported by the Australian Government Department of Health and Ageing. APSU is supported by the NHMRC (Enabling Grant No: 402784 and Practitioner Fellowship No: 457084, E. Elliott), NHMRC Career Development Fellowship (No. 1016272 H. Marshall), the Discipline of Paediatrics and Child Health and

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