The Efficacy and Effectiveness of Influenza Vaccination among Thai Elderly Persons Living in the Community+

Objective: To determine the efficacy and cost-effectiveness of influenza vaccination in the Thai elderly living in an urban community. Material and Method: The study design was a stratified, randomized, double blind, placebo-controlled trial. A total of 635 participants aged 60 years and older living in an urban community was randomized to receive an influenza vaccine or tetanus toxoid as a placebo injection. All participants were followed up 4-6 weeks in the community for influenza-like illness and treatment received, hospitalization and death for one year. A hemagglutination inhibition (HI) test for influenza virus antibody of all participants was done on the day of vaccination as well as 1 month, 5 months, and 12 months after the vaccination. Main outcome measures were immune response rate and protective titer, influenza-like illness, serological influenza, treatment received for influenza-like illness and their expenses, hospitalization and death during the study period. Results: The immune response rate of vaccinations was 97.1 % and protective titer for A (H1N1) and A (H3N2) strains were 96.4 and 98.6%, respectively. The incidence of influenza-like illness was 4.83 % in the vaccine group compared with 10.88 % in the placebo group. The relative risk reduction was 56 % (95%CI = 14 to 77%). The survival analysis also showed that vaccinations significantly reduced the incidence of influenza (p = 0.009). The number needed to prevent one episode was 17 persons (95%CI = 9 to 71 persons)