An application of hidden Markov models to French variant Creutzfeldt-Jakob disease epidemic

Chadeau-Hyam et al have a new paper in Applied Statistics (JRSS C). This is concerned with modelling vCJD in France. A 5 state multi-state model is assumed, with states representing susceptible to infection, asymptomatic infection, clinical vCJD, death from vCJD and death from causes other than vCJD. The data available are extremely sparse since no reliable test is available to distinguish susceptible from asymptomatic. Indeed the only data actually observed are the yearly transitions from infected to clinical vCJD and clinical vCJD to death. As a result, pseudo-observed quantities, estimated in previous studies or from general population data are used to get quantities such as the numbers susceptible. Various approximations in terms of the number and type of transitions possible by an individual in one year are also made. Some simulations are performed which suggest the results are reasonably robust to these approximations.

The most interesting methodological aspect of the paper is the use of an (approximately) Erlang distribution for the incubation time (rather than an Exponential). This is achieved by assuming that the incubation state is made up of 11 latent phases.