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Seasonal influenza

Influenza (flu) is a viral infection affecting the lungs and airways. Seasonal flu outbreaks occur most often in winter in the UK and peaks between January and March. While seasonal influenza is usually self-limiting, complications include bacterial pneumonia and can be life threatening, especially in older people and those with certain underlying health conditions. Seasonal flu is further complicated by the fact there are two types of influenza affecting people: influenza A and influenza B. The influenza virus is also antigenically unstable, meaning new strains and variants are constantly emerging, requiring a new vaccine to be developed and administered each year.

The annual flu programme is a coordinated and evidence based approach to planning for the demands of flu across England. Within the programme there are two forms of vaccine in use: (i) a trivalent vaccine, protecting against three strains of flu (two strains of type A and one strain of type B); (ii) a quadrivalent influenza vaccine, protecting against four strains of flu (two strains of type A and two strains of type B). Eligible within the routine flu programme are those aged 65 years and over and those aged under 65 in a clinical at-risk group (including pregnant women). In recent years there has been a phased roll-out of a children’s programme, which includes pre-school children aged 2 and 3 years old and will eventually comprise all primary school aged children.

Modelling seasonal influenza is complicated by multiple interacting strains including cross-protection between years and a mismatch between levels of infection and reported cases. However, seasonal influenza models, to date, typically treat each flu season independently, with exposure through natural infection and/or vaccination in the preceding flu season having no impact on the current season epidemiological dynamics. The overall premise of our work is to construct a modelling framework incorporating exposure history (in the previous flu season only), and ascertain if there is a vaccine deployment pattern that would be advantageous versus the currently implemented scheme when accounting for such a mechanism.

We are using available data on historical GP consultation rates for influenza-like-illness to fit our models, in combination with information on vaccine efficacy, vaccine uptake and strain composition of sampled influenza viruses. Ably depicting historical seasonal flu outbreaks will provide the platform for running forward simulations and carrying out a cost-effectiveness analysis across a collection of vaccination strategies. These cost-effectiveness assessments will use economic models that accurately estimate costs to the NHS for each case of influenza, and compare these influenza-linked health outcome expenses to the cost of distributing and administering the influenza vaccine.

Associated publications
  1. Hill EM, Petrou S, de Lusignan S, Yonova I & Keeling MJ. (2019) Seasonal influenza: Modelling approaches to capture immunity propagation. PLoS Computational Biology 15(10): e1007096. doi: 10.1371/journal.pcbi.1007096.

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A 6-year project, funded by the UK Department of Health.

Researchers involved:

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Matt Keeling (Professor, joint between Warwick Mathematics Institute and School of Life Sciences)

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Stavros Petrou (Professor in Health Economics, Warwick Medical School)

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Graham Medley (Professor, London School of Hygiene and Tropical Medicine)

Sophie Staniszewska

Sophie Staniszewska (Leader of Patient and Public Involvement, Warwick Medical School)

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Martin Underwood (Director of Clinical Trials Unit, Warwick Medical School)

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Ed Hill (Postdoctoral researcher, Warwick Mathematics Institute)