Assessment of Viral Shedding in Children Previously in Receipt of Multiple Doses of Live Attenuated Influenza Vaccine (LAIV) Compared to Influenza Vaccine-naïve Controls

LAIV shedding studies in children could be an important way to confirm whether impediments to viral replication do indeed explain these observed reductions in vaccine effectiveness(VE), whether prior vaccination has any influence on replication and what future implications (if any) this might have for the UK paediatric LAIV programme. LAIV virus replication in children will be dependent on virological and host factors. The virus factors include replicative fitness of individual strains and the susceptibility to inhibition by other replicating strains (ability to compete). Host factors which may influence this include pre-existing specific immunity as a result of prior infection or previous vaccination (with either LAIV or IIV), and innate immune factors including mucosal immunity. Understanding the relative importance of different factors over two seasons when the strain composition of the A/H1N1pdm09 LAIV virus will change and by comparing previously unvaccinated and highly vaccinated groups (with both LAIV and IIV), can potentially give unique insights into their contribution to the US LAIV observations. With the change of the A/H1N1pdm09 vaccine strain in 2017/18, demonstrating improved performance (in terms of VE, virus shedding and immunogenicity) and what contribution prior vaccination might make will be key evidence for both the UK, but also the US. Information presented at the ACIP in June 2018 from the 2016/17 and 2017/18 seasons will be key to inform US future decisions around use of LAIV. This is a parallel group, non randomised study which will enrol at least 400 children. Both written informed consent from parent/ guardian and written assent from the child will be in place prior to any study procedure. The two groups will be defined by previous influenza vaccination history, with around half the children naïve to any influenza vaccination (LAIV or IIV) and half having had at least three doses of LAIV with or without IIV. All will follow the same sched