  We study three different strategies of vaccination in a SEIRS
(Susceptible--Exposed--Infected--Recovered--Susceptible) seasonal forced model,
which are: ($i$) continuous vaccination; ($ii$) periodic short time localized
vaccination and ($iii$) periodic pulsed width campaign. Considering the first
strategy, we obtain an expression for the basic reproduction number and infer a
minimum vaccination rate necessary to ensure the stability of the disease-free
equilibrium (DFE) solution. In the second strategy, the short duration pulses
are added to a constant baseline vaccination rate. The pulse is applied
according to the seasonal forcing phases. The best outcome is obtained by
locating the intensive immunization at inflection of the transmissivity curve.
There, a vaccination rate of $44.4\%$ of susceptible individuals is enough to
ensure DFE. For the third vaccination proposal, additionally to the amplitude,
the pulses have a prolonged time width. We obtain a non-linear relationship
between vaccination rates and the duration of the campaign. Our simulations
show that the baseline rates, as well as the pulse duration, can substantially
improve the vaccination campaign effectiveness. These findings are in agreement
with our analytical expression. We show a relationship between the vaccination
parameters and the accumulated number of infected individuals, over the years
and show the relevance of the immunisation campaign annual reaching for
controlling the infection spreading. Regarding the dynamical behaviour of the
model, our simulations shows that chaotic and periodic solutions, as well as
bi-stable regions, depend on the vaccination parameters range.
