Influenza modelling and preparation for outbreaks

Equine influenza is a highly contagious disease which, although clinical signs are usually self-limiting, has the potential to be highly disruptive to training and competition schedules. As a result, vaccination against equine influenza is compulsory for many competition horses. For more than 2 decades, the Horserace Betting Levy Board has supported a programme of research, including modelling, aimed at optimising prevention and control of equine influenza outbreaks.
Mathematical modelling is augmenting rather than replacing epidemiological studies and animal experiments, but it is increasingly considered an important tool for development of disease prevention and control measures. In many instances, modelling may simply seem to provide confirmation of the obvious, but it is nonetheless important that decisions about vaccination regimens, for example, are evidence-based. Conclusions drawn from mathematical models must always be critically evaluated; a major limitation is that they are only as good as the available data used to inform them. Where data are lacking, assumptions have to be made and preconceived ideas may lead to conceptual errors in the construction of a model. On the other hand, the requirement for accurate data to inform models leads to available data being critically reviewed, which may reveal novel findings that might otherwise have been overlooked. Modelling has demonstrated that introduction of equine influenza into a vaccinated population usually leads to infection of only a small proportion of animals (<5%) rather than ‘spreading like wildfire’ as it would in a naïve population. Mathematical modelling has particular value in extrapolating data from small experimental studies to the population level. For example, mathematical modelling demonstrated that a small difference in duration of infectiousness at the individual level resulting from use of a ‘mismatched’ vaccine can have a significant impact on the likelihood of a large outbreak taking off at the population level.
Earlier mathematical models focused on studying the transmission of virus between horses housed in an enclosed population (at the yard level). Most of these studies are static or only consider the risk of outbreaks occurring over a relatively short time-frame. Increasingly, models are examining the dynamics of interactions (including those occurring within hosts) and incorporating human factors that may influence how disease spreads and the outcome of disease control measures.