ABSTRACT
This study aimed to determine the effectiveness of influenza vaccinations among the elderly in Bangkok in reducing influenza-like illness (ILI) and influenza-related complications. Using a non-randomized, controlled, prospective methodology, healthy, active people aged 60 years or more, living in the Bangkok Metropolitan Administration (BMA) area, were studied. The two study cohorts comprised 519 persons in the vaccinated group and 520 in the non-vaccinated group. The outcome under study was influenza-like illness (ILI), as reported by the study volunteers. The two groups were comparable for most socio-demographic characteristics, except for gender, level of education, marital status, and smoking habit. The age range was 60-88 years (mean: 68 years). Females outnumbered males in both groups, with ratio of female to male of 2.6:1 and 1.9:1 in the vaccinated and non-vaccinated groups, respectively. The top three co-morbidities among these groups were hypertension, diabetes mellitus, and heart disease, in that order. Only 1% of the volunteers reported lung disease as co-morbidity. During the 12-month study period, a total of 107 volunteers reported ILI in both groups, with 38 persons in the vaccinated group and 69 persons in the non-vaccinated group. There were 46 ILI episodes in the vaccinated group, and 86 in the non-vaccinated group, for a total of 132 episodes. The incidence rates rates of influenza in this population, therefore, were 8.9% for the vaccinated and 16.9% for the non-vaccinated groups; with a reduction in the rate of reported ILI and doctor visits of 8%. Vaccine effectiveness was rated at 47.6%, crude risk ratio at 1.9 (1.33-2.75), and adjusted risk ratio at 1.92 (95% CI: 1.25-2.95), after adjustment for gender, marital status, education, and smoking habit. No complications due to ILI were observed in this population during the study period. Hospitalizations during this period were due to non-ILI related causes, such as cancer and accident.