Incidence of influenza and influenza vaccine effectiveness in the 2004-2005 season

Abstract

Background. To quantify the incidence of flu in different groups of the population of Navarra in the 2004-2005 season, and to evaluate the effectiveness of anti-flu vaccination. Methods. The analysis of influenza cases in the system of compulsory notificable diseases was complemented by the individualised notifications in the network of sentinel doctors that covers a population of 22,339 inhabitants. The coverage and effectiveness of the vaccine was studied. Results. Vaccine coverage in ³65 year olds reached 62%. The incidence of influenza was 42.6 cases per 1,000 inhabitants. It reached a maximum in mid-January, exceeding 750 weekly cases per 100,000 inhabitants and 1,900 cases per 100,000 children. The highest rate of influenza was observed in under-15 year olds (49.4 cases per 1,000 inhabitants) and the lowest in non-institutionalised ³65 year olds (2.6 per 1,000 inhabitants), although it was higher in geriatric residences (62.1 per 1,000; p<0.0001). Seventy-nine percent of the cases from 5 to 64 years resulted in absenteeism from school or work. The flu virus was identified in 42/65 (65%) nasopharyngeal smears, 90% being influenza virus A(H3). The incidence of influenza was 3.08% in the unvaccinated and 0.45% in the vaccinated (p<0.001). The global effectiveness of the anti-flu vaccine was 65%, and in ³65 years old it was 73%. Conclusion. Although its effectiveness is not total, the vaccine is the main measure for preventing influenza. The network of sentinel doctors provide useful information for the coordination of care and public health activities against flu.