  This paper presents a simple continuous-time linear vaccination-based control
strategy for a SEIR (susceptible plus infected plus infectious plus removed
populations) propagation disease model. The model takes into account the total
population amounts as a refrain for the illness transmission since its increase
makes more difficult contacts among susceptible and infected. The control
objective is the asymptotically tracking of the removed-byimmunity population
to the total population while achieving simultaneously the remaining population
(i.e. susceptible plus infected plus infectious) to asymptotically converge to
zero. A state observer is used to estimate the true various partial populations
of susceptible, infected, infectious and immune which are assumed to be
unknown. The model parameters are also assumed to be, in general, unknown. In
this case, the parameters are replaced by available estimates to implement the
vaccination action.
