Influenza-related deaths and hospitalizations in Hong Kong: A subtropical area

Summary

Background

While influenza vaccination has been widely used in developed countries to reduce mortality and morbidity in high-risk populations, the lack of regional data on the health burden of influenza and the uncertainty of the applicability of data from temperate areas have been major impediments to establishing an evidence-based policy on the wider use of influenza vaccine in tropical and subtropical regions of the world.

Objectives

The aim of this study was to estimate the annual excess deaths and hospitalizations related to influenza in the Hong Kong Special Administrative Region (HKSAR) for the years 1999 and 2000.

Study design and methods

Correlation and regression models were used to estimate the excess deaths and hospitalizations related to influenza in the general population of HKSAR for the years 1999 and 2000, using routinely collected mortality and hospitalization data, and virological laboratory data collected by the HKSAR Influenza Surveillance System.

Results

The annual mean excess numbers of deaths related to influenza in Hong Kong were estimated to be 613 for pneumonia and influenza, and 2302 for respiratory and circulatory diseases. The mean excess numbers of hospitalizations attributable to influenza were 4051 for pneumonia and influenza, and 15,873 for respiratory and circulatory diseases. The crude influenza-related mortality and hospitalization rates in Hong Kong, a subtropical area, exceeded those documented in temperate regions.

Conclusions

The finding of significant mortality and morbidity related to influenza in a subtropical area is in accordance with the results of previous studies in tropical and subtropical regions. This simple methodology can be used for the development of influenza immunization policy in many developing countries in tropical and subtropical regions. The enormous potential of influenza vaccination in saving lives and reducing suffering warrants serious consideration of the expanded use of influenza vaccine in tropical and subtropical regions.

Introduction

While the significant mortality and morbidity caused by influenza have been well documented and established in many developed countries,1, 2, 3 its burden and public health significance in the developing world, particularly in tropical and subtropical regions, are largely unknown.4, 5 Apart from a lack of the necessary infrastructure and resources for adequate disease reporting and virological surveillance in many of these countries, the usual occurrence of influenza transmission throughout the year,⁶ instead of well-defined seasonal outbreaks as in temperate regions, makes it more difficult to assess the health impact caused by influenza. Unfortunately, this lack of influenza health burden data has generally led to the perception that influenza is not an important health problem and does not have a significant health impact in the populations in developing countries in tropical and subtropical regions.5, 7, 8, 9 The result is that while most developed countries throughout the world have implemented influenza vaccination programmes for high-risk populations to reduce influenza-related mortality and morbidity, this proven effective and life-saving preventive measure has not been practised in most, if not all, developing countries.4, 5, 10 The recent outbreak of influenza in Madagascar¹¹ in 2002, resulting in 27,000 cases and 750 deaths over 3 months despite rapid intervention, may well serve as a ‘wake-up call’ against this complacency. In its Global Influenza Programme (2002), the World Health Organization (WHO) identified the expanded use of influenza vaccines in developing countries as a key objective, and the evaluation of disease burden in countries where there is no recognition of influenza or no control policies are in place as a priority activity.¹² This need has also been identified and advocated by many public health practitioners over the years.5, 10, 13, 14

While various techniques2, 13, 15 have been used to estimate excess mortality and morbidity attributable to influenza, complex methodologies requiring extensive epidemiologic and laboratory data and statistical expertise are usually not practical or feasible in developing countries. This study estimated the mortality and hospitalizations related to influenza in the population of the Hong Kong Special Administrative Region (HKSAR) using routine and existing vital statistics, and medical service and laboratory data. The objectives of this paper were to delineate the health burden of influenza in Hong Kong where such information is relatively lacking, and to propose a simple mathematical methodology that can be applied in other tropical and subtropical countries for similar purposes.