  In this paper, we investigate two major immunization strategies, random
immunization and targeted immunization, of the susceptible-infected (SI) Model
on the Barab\'{a}si-Albert (BA) networks. For the heterogenous structure, the
random strategy is quite ineffective if the vaccinated proportion is quite
small, while the targeted one which prefers to vaccinate the individuals with
the largest degree can sharply depress the epidemic spreading even only a small
amount of population are vaccinated. The analytical solution is also
obtained,which can capture the trend of velocity change versus the amount of
vaccinated population.
