Risk factors for admission to hospital with laboratory-confirmed influenza in young children: birth cohort study.

We determined risk factors for influenza hospital admission in children aged <2 years to guide the design of paediatric vaccination programmes.We linked all singleton live births in Scotland from 2007 to 2015 to hospital administrative data and influenza laboratory reports. Cox proportional hazard models were used to identify birth and family risk factors for influenza admissions.There were 1115 influenza admissions among 424 048 children. 85.1% of admitted children were born at term and were not in a high-risk group. Presence of an older sibling was strongly associated with increased risk of influenza admission, particularly for children aged <6 months: hazard ratio for second- versus first-born child was 2.02 (95% CI 1.52-2.69). Maternal age <30 years and birth during autumn (age <6 months) or spring (age 6-23 months) were also associated with admission risk.Targeting vaccination programmes to high-risk children will not prevent the vast majority of influenza admissions. Parents of children aged <2 years should be advised that vaccination of older siblings will protect younger children against influenza infection. As evidence of the impact of the universal influenza vaccine programme emerges, there may be a need to reconsider universal influenza vaccination in children aged 6 months to 2 years in the UK.