Establishing seasonal and alert influenza thresholds in Morocco

Ahmed Rguig,Imad Cherkaoui,Zakia Regragui,Soumia Triki,Houria Elmbarki,Chakib Nejjari,Margaret Mccarron,Mohammed Youbi,Hicham Oumzil,Hassan Ihazmad,Abderrahman Bimouhen,Fatima El Falaki

doi:10.1186/s12889-020-09145-y

Abstract

BackgroundSeveral statistical methods of variable complexity have been developed to establish thresholds for influenza activity that may be used to inform public health guidance. We compared the results of two methods and explored how they worked to characterize the 2018 influenza season performance–2018 season.MethodsHistorical data from the 2005/2006 to 2016/2018 influenza season performance seasons were provided by a network of 412 primary health centers in charge of influenza like illness (ILI) sentinel surveillance. We used the WHO averages and the moving epidemic method (MEM) to evaluate the proportion of ILI visits among all outpatient consultations (ILI%) as a proxy for influenza activity. We also used the MEM method to evaluate three seasons of composite data (ILI% multiplied by percent of ILI with laboratory-confirmed influenza) as recommended by WHO.ResultsThe WHO method estimated the seasonal ILI% threshold at 0.9%. The annual epidemic period began on average at week 46 and lasted an average of 18 weeks. The MEM model estimated the epidemic threshold (corresponding to the WHO seasonal threshold) at 1.5% of ILI visits among all outpatient consultations. The annual epidemic period began on week 49 and lasted on average 14 weeks. Intensity thresholds were similar using both methods. When using the composite measure, the MEM method showed a clearer estimate of the beginning of the influenza epidemic, which was coincident with a sharp increase in confirmed ILI cases.ConclusionsWe found that the threshold methodology presented in the WHO manual is simple to implement and easy to adopt for use by the Moroccan influenza surveillance system. The MEM method is more statistically sophisticated and may allow a better detection of the start of seasonal epidemics. Incorporation of virologic data into the composite parameter as recommended by WHO has the potential to increase the accuracy of seasonal threshold estimation.

Highlights

Several statistical methods of variable complexity have been developed to establish thresholds for influenza activity that may be used to inform public health guidance
As a complement to the thresholds using syndromic data, we calculated a threshold using a composite parameter integrating both syndromic and virologic surveillance data. Following these direct comparisons of the methodologies, we explored the best method for characterizing the 2017/ 2018 influenza activity
Average influenza like illness (ILI) activity thresholds: World Health Organization (WHO) methodology When applying the WHO method to our 11 years of surveillance data, we estimated that the seasonal threshold was the point at which more than 0.9% of outpatient consultations were due to ILI (Table 2)

Summary

Introduction

Several statistical methods of variable complexity have been developed to establish thresholds for influenza activity that may be used to inform public health guidance. Seasonal influenza epidemics result in considerable annual morbidity and mortality, with an estimated 291,243 to 645,832 deaths per year globally [1]. Associated with these seasonal epidemics are substantial economic losses due to absenteeism, lost wages and increased utilization. Local patterns of influenza virus circulation and seasonality may differ geographically, necessitating national estimates of seasonal influenza activity to inform public health guidance. Establishing baseline activity, epidemic and alert thresholds is a useful tool to inform recommendations for timely influenza vaccination to lessen the burden of seasonal epidemics [4]

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