  In a world being hit by waves of COVID-19, vaccination is a light on the
horizon. However, the roll-out of vaccination strategies and their influence on
the pandemic are still open problems. In order to compare the effect of various
strategies proposed by the World Health Organization and other authorities, a
previously developed SEIRS stochastic model of geographical spreading of the
virus is extended by adding a compartment for vaccinated people. The parameters
of the model were fitted to describe the pandemic evolution in Argentina,
Mexico and Spain to analyze the effect of the proposed vaccination strategies.
The mobility parameters allow to simulate different social behaviors (e.g.
lock-down interventions). Schemes in which vaccines are applied homogeneously
in all the country, or limited to the most densely-populated areas, are
simulated and compared. The second strategy is found to be more effective.
Moreover, under the current global shortage of vaccines, it should be remarked
that immunization is enhanced when mobility is reduced. Additionally,
repetition of vaccination campaigns should be timed considering the immunity
lapse of the vaccinated (and recovered) people. Finally, the model is extended
to include the effect of isolation of detected positive cases, shown to be
important to reduce infections.
