Incidence of Influenza-Like Illness and Severe Acute Respiratory Infection during Three Influenza Seasons in Bangladesh, 2008-2010/incidence Du Syndrome De Type Grippal et Infection Respiratoire Aigu Severe Lors Des Trois Saisons De la Grippe Au Bangladesh, De 2008 a 2010/incidencia del Sindrome Seudogripal Y De la Infeccion Respiratoria Aguda Grave Durante Tres Temporadas De Gripe En Bangladesh, Entre Los Anos 2008 Y 2010

Introduction Influenza is believed to be an important cause of morbidity and mortality worldwide but data about its burden in low-income tropical countries like Bangladesh are rarely available. Data on incidence as well as on risk factors are valuable in prioritizing influenza prevention and control efforts in the light of competing health interventions. (1,2) Data documenting a high incidence of influenza in the United States of America and Europe has helped guide investments in influenza control and prevention in high-income countries. (3-7) However, there is little information about the incidence of influenza in low-income tropical countries. (8,9) According to health authorities, the 2009 pandemic influenza A (H1N1) virus caused approximately 6000 deaths in Bangladesh (10) and cost Dhaka 6.1 million United States dollars (US$) in direct medical costs to the patient. (11) Infrequent hand washing and poor respiratory hygiene (e.g. covering mouth when coughing), (12) limited access to care, lack of awareness of antiviral treatment and its availability, a huge shortage of influenza vaccines early in the pandemic, and a high prevalence of malnutrition (13) may have worsened the pandemic in low-income countries such as Bangladesh. (14) In general, the rate of hospitalization associated with seasonal and pandemic influenza in low-income tropical and subtropical countries remains largely unknown. In 2004, the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR,B) initiated influenza surveillance in a population-based site among children aged less than 5 years in urban Dhaka. The incidence of laboratory-confirmed influenza infection in the site during 2004-2007 was estimated at 10.2 per 100 person-years among children aged less than 5 years seeking care at ambulatory clinics. (15) Beginning in April 2007, the Government of Bangladesh and ICDDR,B initiated national hospital-based influenza surveillance with a network of 12 surveillance hospitals throughout the country to provide nationally-representative data from all age groups. Although this national surveillance system helped to establish the seasonality of influenza, which typically occurs during the monsoon season from May to September, (16) the incidence of influenza could not be estimated because people in hospital catchment areas sought care not just in hospitals but in other facilities as well and data on the population at risk, needed for the denominator, were not available. In the present study, we used health utilization patterns in the catchment areas of four selected sentinel hospitals to estimate the incidence of both outpatient and hospital visits for seasonal and pandemic influenza illness during three influenza endemic periods during 2008-2010. Methods Enhanced hospital-based surveillance Investigators selected one private and three government hospitals, each in a different region of Bangladesh. The four hospitals, which were participating in the 12-hospital Government of Bangladesh/ICDDR,B national hospital-based influenza surveillance programme, were Jahurul Islam Medical College Hospital in Kishorgonj; Comilla Medical College Hospital in Comilla; Shahid Ziaur Rahman Medical College Hospital in Bogra, and Sher-e-Bangla Medical College Hospital in Barisal. Beginning in May 2008, surveillance physicians identified patients with influenza-like illness (ILI, defined as sudden onset of subjective fever, cough or sore throat) in the outpatient departments at sentinel sites during two randomly selected days every month. Investigators only selected patients who resided in the subdistricts belonging to the hospital's main catchment areas (i.e. where the majority of hospital patrons lived), as determined by reviewing hospital log books. Beginning in January 2009, also during two randomly selected days a month, surveillance physicians identified all cases of severe acute respiratory illness (SARI) among children less than 5 years old in sentinel site paediatric wards. …