Abstract

We developed a stochastic SEIR model for the COVID -19
epidemic at a fine spatial scale, using mobile phone mobility data, to describe
the geographical spread of the virus. The model is developed for the very early
phase of the epidemic, when movement of individuals plays a key role. In the
beginning, importation of the virus into the region of interest is decisive,
and we estimate the proportion of unknown imported cases. Our model represents
non-pharmaceutical interventions to contain the epidemic by means of a
regionally varying step function of the effective reproduction numbers. The
regionally varying effective reproduction numbers are estimated by sequential
Approximate Bayesian Computing, using hospitalisation data of the infected
individuals at regional level. For prediction, we develop a way to regularise
the mobility matrices, to conserve the geographical distribution of the
population. This allows adequate long term predictions of all quantities of
interest. Uncertainty in the parameters (both the estimated ones and the ones
learned from the literature) is prolonged into the future by simulation. We use
our model to describe the history of the first phase of the COVID -19 epidemic
in Norway, during which social distancing and hygienic measures have been
adopted together with teleworking and school closure and reopening. The result
of these measures have reduced the presence of the virus in Norway to such a
low level, leading to a relaxation of restrictions, to resemble the early phase
of the epidemic, making our model again important. We compare the results of
our model to the ones obtained by a similar nonregional model. We also
developed a version of the model which has a time varying reproduction number.
In this case we resort to Sequential Monte Carlo for inference. I will discuss
the difficulties in making predictions using this model. This is joint work with Birgitte Freiesleben de Blasio, Solveig
Engebretsen, Gunnar Isaksson Rø, Alfonso Diz-Lois Palomares, Kenth Engø-Monsen,
Anja Bråthen Kristoffersen and Geir Storvik.