Epidemiological Characteristics and Underlying Risk Factors for Mortality during the Autumn 2009 Pandemic Wave in Mexico

Gerardo Chowell,Cécile Viboud,Cesar González-Bonilla,Víctor H Borja-Aburto,Santiago Echevarría-Zuno,Mark A Miller,Irma Fernández-Gárate,Lone Simonsen,Benjamin J Cowling

doi:10.1371/journal.pone.0041069

Abstract

BackgroundElucidating the role of the underlying risk factors for severe outcomes of the 2009 A/H1N1 influenza pandemic could be crucial to define priority risk groups in resource-limited settings in future pandemics.MethodsWe use individual-level clinical data on a large series of ARI (acute respiratory infection) hospitalizations from a prospective surveillance system of the Mexican Social Security medical system to analyze clinical features at presentation, admission delays, selected comorbidities and receipt of seasonal vaccine on the risk of A/H1N1-related death. We considered ARI hospitalizations and inpatient-deaths, and recorded demographic, geographic, and medical information on individual patients during August-December, 2009.ResultsSeasonal influenza vaccination was associated with a reduced risk of death among A/H1N1 inpatients (OR = 0.43 (95% CI: 0.25, 0.74)) after adjustment for age, gender, geography, antiviral treatment, admission delays, comorbidities and medical conditions. However, this result should be interpreted with caution as it could have been affected by factors not directly measured in our study. Moreover, the effect of antiviral treatment against A/H1N1 inpatient death did not reach statistical significance (OR = 0.56 (95% CI: 0.29, 1.10)) probably because only 8.9% of A/H1N1 inpatients received antiviral treatment. Moreover, diabetes (OR = 1.6) and immune suppression (OR = 2.3) were statistically significant risk factors for death whereas asthmatic persons (OR = 0.3) or pregnant women (OR = 0.4) experienced a reduced fatality rate among A/H1N1 inpatients. We also observed an increased risk of death among A/H1N1 inpatients with admission delays >2 days after symptom onset (OR = 2.7). Similar associations were also observed for A/H1N1-negative inpatients.ConclusionsGeographical variation in identified medical risk factors including prevalence of diabetes and immune suppression may in part explain between-country differences in pandemic mortality burden. Furthermore, access to care including hospitalization without delay and antiviral treatment and are also important factors, as well as vaccination coverage with the 2008–09 trivalent inactivated influenza vaccine.

Highlights

A number of researchers have explored the clinical and epidemiological characteristics of novel A/H1N1 influenza in different populations around the globe (e.g., [1,2,3,4,5,6,7,8,9,10,11,12]), but analyses of large individual-level clinical data spanning multiple geographic regions and disease severity outcomes of A/H1N1 infections are scarce
Hospital admission delay and length of stay Out of the 10435 acute respiratory infection (ARI) hospitalizations recorded during the fall pandemic wave, the number of A/H1N1-positive hospitalizations and inpatient deaths were 2944 and 517, respectively (Figure 1)
The average time from symptoms onset to admission among A/ H1N1-positive inpatients was 3.1 days while the average length of hospital stay was 5.3 days, but it was significantly longer for A/H1N1 inpatients with fatal outcomes than for those who recovered (4.9 days vs. 7.7, Wilcoxon test, P,0.0001)

Summary

Introduction

A number of researchers have explored the clinical and epidemiological characteristics of novel A/H1N1 influenza in different populations around the globe (e.g., [1,2,3,4,5,6,7,8,9,10,11,12]), but analyses of large individual-level clinical data spanning multiple geographic regions and disease severity outcomes of A/H1N1 infections are scarce These studies could be crucial to quantify the role of underlying population health, case management, hospital admission delays and potential changes in the influenza virus characteristics on the mortality burden of the 2009 A/H1N1 influenza pandemic across countries [13]. Elucidating the role of the underlying risk factors for severe outcomes of the 2009 A/H1N1 influenza pandemic could be crucial to define priority risk groups in resource-limited settings in future pandemics

Methods

Results

Discussion

Conclusion