The United Kingdom (UK) has a long-standing influenza selective immunisation programme offering inactivated vaccine to people 65 years of age and older and those 6 months to less than 65 years of age with an underlying clinical risk factor [1]. Following advice from the Joint Committee on Vaccination and Immunisation (JCVI), the UK started the incremental introduction of a universal childhood influenza vaccine programme in the 2013/14 influenza season [2] with a newly licensed intra-nasally administered live attenuated influenza vaccine (LAIV). Eligible healthy children were offered a single dose of LAIV, whereas children in a clinical risk group up to 9 years of age, with no contraindications for LAIV and not previously vaccinated, were offered two doses of vaccine. By the 2016/17 season, all children aged 2–8 years across the UK were being offered quadrivalent LAIV (LAIV4), or else quadrivalent inactivated vaccine (QIV) if LAIV4 was contraindicated [1]. In addition, Scotland and Northern Ireland offered LAIV4 also to all remaining children of primary school age up to 11 years of age. The UK has found evidence of LAIV4 effectiveness in 2015/16 of 58% and continues to recommend its use [3]. This is in contrast to the United States (US) where there has been a longstanding paediatric influenza vaccination programme, and where reduced LAIV vaccine effectiveness (VE) has been described by the US Centres for Disease Control (CDC). This led to a recommendation to suspend use of LAIV in children in 2016/17 [4] and important questions about what might explain these observations of apparent reduced LAIV effectiveness, including what role prior vaccination may play [5].

The 2016/17 influenza season in the UK, as with many other northern hemisphere countries, was characterised by the early circulation of influenza A(H3N2) viruses. The season started in December 2016 and peaked over the Christmas/New Year period. It was characterised by large numbers of care home outbreaks, many of which included highly vaccinated populations, increased admissions to hospital compared with the previous season and significant excess mortality particularly among those 65-year-old or older, despite vaccine uptake levels of?over?70% in this age group [6]. Questions have been previously raised about the effectiveness of inactivated influenza vaccine in older persons and a range of potential explanatory factors have been postulated including what role prior vaccine exposure may play in reducing VE in this age group [7].

The UK has a well-established system to monitor influenza VE each season, including mid-season estimates based upon sentinel swabbing in primary care [3,8]. Here we present the 2016/17 end-of-season VE findings for laboratory-confirmed infection in primary care across all age groups, with a particular focus on LAIV4 in children and inactivated influenza vaccine (IIV) in adult age groups and we explore the possible effect of prior season vaccination on VE in the current season. A comparison with the mid-season estimate is also undertaken.