There are few studies on the effectiveness of influenza trivalent inactivated vaccine in high-risk children. Since 2008, the Western Australian government has offered free trivalent inactivated vaccine immunisation for children aged 6–59 months, facilitating a large case control study.1 Children aged 6–59 months presenting with an influenza-like illness to the main children's hospital Emergency Department from 2008 to 2014 (excluding the H1N1 epidemic year 2009) were tested for influenza infection by nasopharyngeal culture or polymerase chain reaction. Reported immunisation status was verified from national and doctor records. Vaccine efficacy (VE) was calculated by comparing the vaccine status of 546 influenza positive children with that of 2177 negative children. Overall, VE was 70.0% (95% confidence interval (CI) 47.7–82.9%). Surprisingly, there was a non-significant trend for VE to be higher for children <2 years old = 84.7% (49.6–95.3%), children born preterm (<37 weeks) = 79.2% (95%CI 10.9–95.1%) and children with defined medical co-morbidities at risk of severe influenza = 82.5% (95%CI 14.6–96.4%), the very groups in which reduced VE was expected. However, absolute numbers were low for these groups. Influenza vaccine efficacy varies from year to year, depending on the match between vaccine and circulating strains. These data show promisingly high efficacy for those children at the highest risk of severe influenza and should encourage progress towards universal childhood immunisation.