Influenza epidemiology, vaccine coverage and vaccine effectiveness in sentinel Australian hospitals in 2012: the Influenza Complications Alert Network (FluCAN)

Abstract

Influenza is mostly a mild, self-limiting infection and severe infection requiring hospitalisation is uncommon. Immunisation aims to reduce serious morbidity and mortality. The Influenza Complications Alert Network (FluCAN) is a sentinel hospital-based surveillance program that operates at 15 sites across all states and territories in Australia. This study reports on the epidemiology of hospitalisation with confirmed influenza, estimate vaccine coverage and influenza vaccine protection against hospitalisation with influenza during the 2012 influenza season. In this observational study, cases were defined as patients admitted to one of the sentinel hospitals with influenza confirmed by nucleic acid detection. Controls were patients who had acute respiratory illnesses who were test-negative for influenza. Vaccine effectiveness was estimated as 1 minus the odds ratio of vaccination in case patients compared with control patients, after adjusting for known confounders. During the period 9 April to 31 October 2012, 1,231 patients were admitted with confirmed influenza at the 15 FluCAN sentinel hospitals. Of these, 47% were more than 65 years of age, 8% were Indigenous Australians, 3% were pregnant and 76% had chronic co-morbidities. Influenza A was detected in 83% of patients. Vaccination coverage was calculated from the vaccination status of 1,216 test negative controls and was estimated at 77% in patients 65 years or over and 61% in patients with chronic comorbidities. Vaccination effectiveness was estimated at 41% (95% CI: 28%, 51%, P<0.001). Vaccine coverage was incomplete in at-risk groups, particularly non-elderly patients with medical comorbidities. The study results suggest that the seasonal influenza vaccine was moderately protective against hospitalisation with influenza during the 2012 season.