Mortality from causes other

than influenza starts from age 65 and thereafter is assumed to be a constant risk, corresponding to a mean life expectancy of 25 years for individuals aged 65 (Table 1). Individuals in different age groups mix with one another as defined in a UK specific age stratified contact matrix developed by the POLYMOD study [16]. Such matrices are usually referred to as ‘Who Acquires Infection from Whom’ (WAIFW) contact matrices (Fig. 1) and provide a relative measure of the frequency learn more of contact between individuals of different or similar ages. An influenza transmission model was developed, building on an approach set out previously [17]. For the purposes of this model, influenza is assumed to occur as two sub-types of influenza A (e.g. H1N1 and H3N2) and as influenza B. All subtypes are assumed to be immunologically distinct and to occur every two years, with the A subtypes alternating to give an annual peak in incidence between week 40 and week 20 of the following year. The dynamic transmission model subdivides the population into 5 subgroups, the Susceptible, Exposed, Infectious, Recovered and Vaccinated populations (Fig. 2). This stratification is based on the influenza virus infection status of members of the population

and not on clinical presentation. A set of linked differential equations (see Appendix A) describes the flow of individuals between these subgroups and the system is solved numerically using a fourth order Runge–Kutta method with adaptive step control [18]. Exposed (latently infected) individuals are assumed to be infected for an average of 2 days before becoming infectious this website [19]. They remain infectious on average for a further 2 days [19], during which time the intensity and duration of viral shedding is assumed to be uniform across the age bands. SB-3CT Once an individual has recovered from infection, they are assumed to be immune to reinfection with the same subtype. This immunity wanes over time as a result of the combined effects of a gradual decline in immunological memory and antigenic drift on the part of the virus. The resulting duration

of protection was assumed to last for 6 and 12 years for influenza A and influenza B, respectively [17]. The basic reproductive rate (R0) is defined as the number of secondary infections arising from one primary infection in a totally susceptible population [20] and [21]. Using data from past pandemics, R0 for influenza has been estimated to range from 1.6 to 3.9 [22] and [23]. A value for the transmission coefficient was chosen, corresponding to a conservative R0 of 1.8, calculated using the dominant eigenvalue of the next generation matrix [24] and [25]. The incidence of influenza follows a marked seasonal pattern. Peak incidence was assumed to occur on December 22 and to reach a minimum on June 23. The magnitude of the basic reproduction number at the peak of influenza incidence compared to baseline was set to 1.43 [17].