Effectiveness of Influenza Vaccination in Ili Prevention

The aims of the study were to determine the attack rate of influenza-like illness among inhabitants of five old folk homes nationwide using influenza vaccine as a probe and the effectiveness of influenza vaccination in prevention of influenza-like illness. We conducted a nonrandomized, single-blind placebo control study from June 2003 to February 2004. VAXIGRIPR 2003 Southern hemisphere formulation was used. Among 527 subjects, the attack rates of influenza-like illness in the influenza vaccine group were 6.4, 4.6 and 2.4% during the first, second and third 2-month periods, respectively. The attack rates of influenza-like illness in the placebo group were 17.7, 13.8 and 10.1%. Influenza vaccination reduced the risk of contracting influenza-like illness by between 14, and 45%. The vaccine effectiveness in reducing the occurrence of influenza-like illness ranged from 55 to 76%, during the 6-month study followup. The presence of cerebrovascular diseases significantly increased the risk of influenza-like illness (p < 0.005). Vaccine recipients had fewer episodes of fever, cough, muscle aches, runny nose (p < 0.001) and experience fewer sick days due to respiratory illness. Subjects who received influenza vaccination had clinically and statistically significant reductions in the attack rate of influenza-like illness. Our data support influenza vaccination of persons with chronic diseases and >50 year olds living in institutions. dromes, disorders affecting the lung, heart, brain, liver, kidneys, and muscles, to fulminant primary viral and secondary bacterial pneumonia. The course is affected by the patient’s age, the degree of pre-existing immunity, properties of the virus, smoking, co-morbidities, immunosuppression, and pregnancy (Nicholson et al, 2003). Persons age 65 years or older are particularly susceptible, experiencing more than 90% of these complications, and also exacerbations of underlying medical conditions, such as chronic heart and lung disease, even death (Glezen 1982; Mc Bean et al, 1993). Annual vaccination is recommended as the mainstay in efforts to prevent influenza in the elderly. The World Health Organization has suggested that elderly individuals be targeted for immunization, but despite this recommendation, influenza vaccine is underused (Nichol INTRODUCTION Influenza virus is unique among respiratory viruses in that epidemics are regularly associated with elevated patient morbidity and mortality, putting enormous pressure on health, social care and other services (Glezen, 1982). Influenza is often associated with elevated rates of pneumonia and hospitalization during epidemics. Influenza virus can cause a broad range of illnesses, from symptomless infection through various respiratory synSOUTHEAST ASIAN J TROP MED PUBLIC HEALTH 842 Vol 38 No. 5 September 2007 et al, 1998). Persistent uncertainties regarding the risks of influenza and the benefits of influenza immunization may contribute to the under utilization of the influenza vaccine. In temperate climates, influenza is characterized by occurrence of annual epidemics during the winter months. Epidemics in tropical countries are less seasonal, and influenza virus can be isolated from significant numbers of patients with respiratory symptoms throughout the year. In Malaysia, influenza virus circulates all year with peaks from June to July and December to January (Nor Shahidah et al, 2003). We conducted a prospective, nonrandomized, single-blind placebo controlled study to determine (1) the incidence of influenza-like illness among inhabitants of old folk homes using influenza vaccine as a probe and (2) the effectiveness of influenza vaccination in prevention of influenza-like illness. MATERIALS AND METHODS