Vaccination in the simulation tool 4Flu

Overview

One of the main purposes of the simulation tool 4Flu is the comparison of influenza vaccination strategies. To do so, the user can customize his or her own vaccination strategies, using either trivalent (TIV) or quadrivalent influenza vaccine (QIV).

Vaccinations

Screenshot of vaccination coverage settings According to the default parameter settings in 4Flu, vaccinations occur annually from 1 October to 30 November (as the simulation year starts in the transmission-free summer, i.e. on July 1st, the vaccination period begins on the 92nd simulation day and ends on the 153rd). Thereby, the user can specify for each combination of age and risk status, which percentage of individuals are vaccinated and whether they receive TIV or QIV. The user can furthermore specify how likely individuals are re-vaccinated who were vaccinated in the previous season. Whereas QIV covers all 4 influenza strains, TIV vaccination only contains one of the two B lineages. In simulation years for which the composition of TIV is known, the recorded TIV composition is also used in the simulations, but for future years, random decisisons determine TIV composition.

Vaccination-derived immunity

The age-dependent vaccine efficacy determines what percentage of vaccinees are vaccinated successfully, whereby the vaccination success is determined independently for each of the three influenza viruses which are covered by TIV or for the four which are included in QIV. If an individual was non-immune against a variant, and has just been successfully been vaccinated against this variant, the individual acquires a temporary immunity against this variant. The average duration of protection is much shorter after vaccination than after infection. As is the case after infection with one of the B lineages, successful TIV vaccination against one of the B lineages can also trigger immunity against the other B lineage which was not contained in TIV ("B lineage cross protection"). If a new drift variant occurs, the vaccine may not be well-matched against this variant ("drift mismatch"). In that case, the age-specific vaccine efficacy values are reduced by a user-specified factor (default: 0.6) to determine the efficacy of the vaccination against the new drift variant.

Comparison of vaccination strategies

As 4Flu is specifically designed to compare vaccination strategies, two different strategies can be specified for the evaluation period of the simulations. Both strategies will run simultaneously on exactly the same population with the same network structure. Among other output, 4Flu will calculate the difference in the cumulative number of infections between strategy 1 and 2 for each single day of the simulation.