Dynamic modelling of infectious diseases: an application to the economic evaluation of influenza vaccination

To evaluate the economic efficiency of influenza vaccination using both dynamic and static modelling approaches.

The Spanish National Health System.

We modelled the progress of an influenza epidemic in Spain according to the epidemiological pattern of susceptible-->infective-->resistant, employing a non-linear system of ordinary differential equations that enables the measurement of epidemiological effects of an anti-influenza vaccination. We used a decision tree to represent the repercussion on healthcare resources use and on financial resources. The same analyses were conducted using a static approach, and the results were compared. Healthcare costs were valued in euro, year 2005 values.

For the base case, the impact of the healthcare intervention (vaccination) was not efficient from the perspective of the healthcare payer when using a static approach (return rate 0.28 per euro invested in vaccination). Nevertheless, it was efficient when employing a dynamic approach (return rate 1.22 per euro). Furthermore, a considerable freeing of healthcare resources would have been produced over the entire influenza season.

The indirect effect of vaccination on the non-vaccinated individuals (the 'herd immunity effect') can be greater than the direct effect on individuals vaccinated. This implies that the herd immunity effect needs to be taken into consideration in the economic evaluations of prophylactic measures employed against infectious diseases.