Changes in characteristics and case-severity in patients hospitalised with influenza A (H1N1) pdm09 infection between two epidemic waves-England, 2009-2010.

Ian D Plumb,Helen K Green,Richard G Pebody,Joanna Ellis,Ross Harris,Kathy Baisley

doi:10.1111/irv.12863

Abstract

BackgroundDuring 2009‐2010, pandemic influenza A (H1N1) pdm09 virus (pH1N1) infections in England occurred in two epidemic waves. Reasons for a reported increase in case‐severity during the second wave are unclear.MethodsWe analysed hospital‐based surveillance for patients with pH1N1 infections in England during 2009‐2010 and linked national data sets to estimate ethnicity, socio‐economic status and death within 28 days of admission. We used multivariable logistic regression to assess whether changes in demographic, clinical and management characteristics of patients could explain an increase in ICU admission or death, and accounted for missing values using multiple imputation.ResultsDuring the first wave, 54/960 (6%) hospitalised patients required intensive care and 21/960 (2%) died; during the second wave 143/1420 (10%) required intensive care and 55/1420 (4%) died. In a multivariable model, during the second wave patients were less likely to be from an ethnic minority (OR 0.33, 95% CI 0.26‐0.42), have an elevated deprivation score (OR 0.75, 95% CI 0.68‐0.83), have known comorbidity (OR 0.78, 95% CI 0.63‐0.97) or receive antiviral therapy ≤2 days before onset (OR 0.72, 95% CI 0.56‐0.92). Increased case‐severity during the second wave was not explained by changes in demographic, clinical or management characteristics.ConclusionsMonitoring changes in patient characteristics could help target interventions during multiple waves of COVID‐19 or a future influenza pandemic. To understand and respond to changes in case‐severity, surveillance is needed that includes additional factors such as admission thresholds and seasonal coinfections.

Highlights

During 2009-2010, pandemic influenza A (H1N1) pdm[09] virus infections in England occurred in two epidemic waves
A Bayesian analysis from available data estimated an increase in overall case-severity in England,[9] consistent with a sentinel surveillance study that noted patients hospitalised during the second wave were nearly twice as likely to be admitted to an intensive care unit (ICU) or to die following infection.[3]
We found that from the first to second epidemic waves there was an approximate two-fold increase in the risk of severe outcomes of ICU admission and death among hospitalised patients

Summary

| BACKGROUND

During 2009-2010, influenza A (H1N1)pdm[09] virus (pH1N1) infections occurred in two distinct epidemic waves in several countries, including in England1-3 and in the United States.4-6 Infections with pH1N1 leading to hospital admissions in England were reported to be more severe during the second wave than during the first wave,[3,7] in contrast to the United States[4,5] and Canada.[8] A Bayesian analysis from available data estimated an increase in overall case-severity in England,[9] consistent with a sentinel surveillance study that noted patients hospitalised during the second wave were nearly twice as likely to be admitted to an intensive care unit (ICU) or to die following infection.[3] It is unclear whether shifts in patient characteristics or health interventions between epidemic waves could have explained the reported increase in clinical severity. We analysed national surveillance data of cases hospitalised during the 2009/2010 pH1N1 pandemic to determine whether reported increased severity of infections among hospitalised patients during the second wave could be explained by changes in reported patient characteristics or health interventions between waves

| METHODS

Findings

| DISCUSSION