In 2013, the NHS started a vaccination program in some regions of England to provide free flu vaccines to children aged 2 years or older, and younger than 16 years. The program has since been rolled out across England. Most children are given a vaccine that is sprayed into their nose. In clinical trials, this vaccine has been shown to protect children from experiencing severe flu symptoms. It is important to describe how it is used, and what happens to children who receive it in the wider community. This evidence will help the NHS to check that the vaccine roll out runs as planned and produces the intended benefits.

This study aims to: (1) describe how many children each year receive flu vaccines, and describe the characteristics of children who are and aren't vaccinated for influenza; (2) test how often children receiving the vaccine see their GP or a hospital doctor for symptoms related to flu, compared to those who don't; and (3) to test what groups of children are more or less likely to receive a flu vaccine. To answer these objectives, the study will use the Clinical Practice Research Datalink, linked to Hospital Episode Statistics and the Office for National Statistics database.

A Live Attenuated Influenza Vaccination (LAIV) has been offered to children through a rolling national program in England since 2013. Previous observational studies identified trends in LAIV uptake that varied with demographic and clinical profiles.

The general aim of this study is to provide up-to-date epidemiological evidence on the uptake of the influenza vaccines, and influenza related HCRU outcomes in children in England. The specific study objectives are given below.

Primary objective:

1. To evaluate the uptake of influenza vaccines in children aged ≥2 and <18 years in England between 2012 and 2020 for LAIV, QIV, and influenza vaccinations where the product is not specified

Secondary objectives:

2.1 To describe usage patterns of LAIV in England between 2012 and 2020. 2.2 To describe HCRU associated with laboratory confirmed influenza and/or influenza-like-illness (ILI) in England between 2012 and 2020 and assess the relative impact of LAIV on HCRU use of LAIV vs no vaccination 2.3 To describe demographics and clinical characteristics of individuals vaccinated with LAIV or not vaccinated against flu by any vaccine in each flu season between 2012 and 2020 2.4 To estimate a DAG aligned model for LAIV uptake in England based on demographic and clinical factors for each flu season and across multiple seasons between 2012 and 2020

The study will include all research eligible aged between ≥2 and <18 years in the study period, with 6 months of history in CPRD before their first influenza vaccination. It will use the Clinical Practice Research Datalink data (vaccinations, diagnoses, characteristics) linked to Hospital Episode Statistics (HCRU). Logistic regression analyses will be used to test for factors associated with vaccine uptake, and outcomes between vaccinated and unvaccinated children. The study will provide up-to-date epidemiological evidence on the uptake of LAIV and its impact on HCRU related to ILI in children in England. This may be used by the NHS for planning and evaluation of the childhood influenza vaccination program.

The reference study cohort will include patients as follows:

• Aged ≥2 and <18 years at any time between 1 September 2012 and 30 April 2020
• Classified as eligible for inclusion in research analysis according to CPRD guidance

In each flu season, the following criteria apply:

Inclusion Criteria:

• Aged ≥6 months and <2 years on the 1st September
• Have a high-risk condition as listed in the Green Book.

Exclusion Criteria:

• Were deceased or had migrated out of their practice prior to the 1st September in the current influenza season.
• Were recorded has receiving both QIV and LAIV in the current influenza season.

The study will include all research eligible children born between the 1st January 1994 and 31st August 2020 (latest date at which a child can be aged 2 years or above in the study period and have at least six months follow up in the most recent data), with 6 months of medical history available in CPRD before their the 1st September in the first year they are aged 2 years or above. The childhood influenza programme has been conducted on a rolling basis since 2013. Classification of each child as eligible for the vaccine will therefore vary year on year depending on their age, GP practice location and presence of high-risk conditions.