Background: Influenza is major cause of paediatric hospitalisation. Influenza vaccine was offered to all children aged 6-59 months resident in Western Australia in 2008, and we wished to evaluate the effectiveness of this immunisation programme.
Objectives: To assess the practicalities of a nested matched case-control design to estimate the protective effect of inactivated influenza vaccination in hospitalised children aged 6-59 months.
Methods: Cases were hospitalised children with laboratory-confirmed influenza, while matched controls were recruited from children admitted for an acute non-respiratory illness. We estimated influenza vaccine effectiveness (VE) against influenza as 1--the adjusted odds ratio from multivariate logistic regression.
Results: The 2008 influenza season was characterised by a late peak and a predominance of influenza virus B. We recruited 26 hospitalised patients with laboratory-confirmed influenza and 50 matched controls. The proportion of cases who were fully vaccinated was 7% versus 30% of controls giving an adjusted VE of 83% (95% CI--54 to 98).
Conclusions: Recruiting sufficient controls was problematic and in the future, we will select controls hospitalised for an influenza-like-illness but influenza negative by laboratory PCR testing. The VE estimate was high but non-significant, reflecting the low number of cases.