Influenza vaccine uptake and its effectiveness in preventing hospitalisations among Australian children with chronic lung diseases

Abstract

Background: Influenza is one of the main viral aetiologies associated with exacerbations of chronic lung diseases (CLDs) in children. Under the Australian Government's Immunisation Program, influenza vaccine is recommended for people aged ≥6 months with chronic medical conditions. However, there are limited data on uptake of influenza vaccine and its effectiveness in Australian children with CLDs. The aim of this study was to determine influenza vaccine uptake and its effectiveness in preventing influenza hospitalisation in children with CLDs using routinely collected administrative data. Methods and materials: We performed a retrospective cohort study using population-based linked administrative data for all children aged ≤17 years with CLDs (including asthma, cystic fibrosis, bronchopulmonary dysplasia and other chronic/congenital lung diseases) born in New South Wales and Western Australia between 2001 and 2012 with follow-up data until 2013. The 10th edition of the International Classification of Disease, Australian Modification codes from hospital datasets were used to identify the study cohort. The main exposure variable was influenza vaccination status obtained from the linked national immunisation registry. The primary outcome was any influenza-coded hospitalisation at least 14 days after vaccination retrieved from the hospital dataset. Cox regression models were used to assess the effectiveness of influenza vaccination in preventing influenza hospitalisation. Results: Of the 1,407,182 children born during 2001 and 2012, 18,954 children (1.4%) had one or more CLDs; 2479 children (13%) with CLDs received one or more doses of influenza vaccine. In total 1,499 children (8%) with CLDs were hospitalised for influenza at least once during the study period. The adjusted incidence/1000 child-years (95% CI) of influenza-associated hospitalisation in vaccinated children with CLD was 54.38 (42.04–69.75) and for unvaccinated children was 30.45 (29.04–31.93). The adjusted hazard ratio of being hospitalised with influenza in vaccinated children compared to unvaccinated children with CLDs was 1.48 (1.14–1.93). Conclusion: This large population-based study has demonstrated low uptake of influenza vaccine in children with CLDs which limits our ability to generate precise vaccine effectiveness estimates. Strategies to improve influenza vaccine uptake and recording of influenza vaccination status may provide better estimates for vaccine effectiveness.