VE estimates were higher when controls included only those children with another respiratory virus detected. Testing for other common respiratory viruses enables the control group to be restricted to those for whom an adequate sample is likely.

Please cite this paper as: Dixon et al. (2010) Lessons from the first year of the WAIVE study investigating the protective effect of influenza vaccine against laboratory‐confirmed influenza in hospitalised children aged 6–59 months. Influenza and Other Respiratory Viruses 4(4), 231–234. 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.