  In this paper we introduce an agent-based model together with a particle
filter approach for studying the spread of COVID-19. Investigations are
performed on the metropolis of Tokyo, but other cities, regions or countries
could have been equally chosen. A novel method for evaluating the effective
reproduction number is one of the main outcome of our approach. Other unknown
parameters and unknown populations are also evaluated. Uncertain quantities, as
for example the ratio of symptomatic / asymptomatic agents, are tested and
discussed, and the stability of our computations is examined. Detailed
explanations are provided for the model and for the assimilation process.
