One of the groups at increased risk of severe influenza illness is children, particularly young children aged five years and below. A national vaccination programme was introduced in England in 2013 to vaccinate children against influenza. Protection against clinically important outcomes is needed to justify resources and can be estimated using observational studies. Analysis of national surveillance data on laboratory-confirmed influenza hospitalisations demonstrated a high and ongoing burden of influenza infection in under five-year-olds in England. It also showed varying severity of influenza by age and influenza subtype and an increase in overall cumulative hospitalisation incidence rates from the 2015/2016 influenza season onwards. In a meta-analysis of influenza vaccine effectiveness studies, influenza vaccination was found to give moderate overall protection to children against hospitalisation. Higher protection was seen with inactivated influenza vaccine, although the difference was not statistically significant, and estimates were higher in seasons when the circulating influenza strains were antigenically matched to the vaccine strains. Two observational study designs often employed to estimate influenza vaccine effectiveness are the test-negative design and the screening method. The strengths and limitations of the two methods were explored through a critical review of the literature. These two methods were then used to estimate vaccine effectiveness against hospitalisation in children in England in the 2013/2014 to 2015/2016 seasons. Both found that the influenza vaccine offered moderately good protection against hospitalisation to children in these seasons. This Research Project provides some key insights into the impact of influenza in children in England and contributes further to the literature on the effectiveness of influenza vaccination against important clinical outcomes in children. It provides useful evidence for other settings considering vaccinating children against influenza as well as methodological insights for assessments of the effectiveness of other vaccines such as COVID-19.