We considered a mathematical model describing the propagation of an epidemic
on a geographical network. The initial growth rate of the disease is the
maximal eigenvalue of the epidemic matrix formed by the susceptibles and the
graph Laplacian representing the mobility. We use matrix perturbation theory to
analyze the epidemic matrix and define a vaccination strategy, assuming the
vaccination reduces the susceptibles. When the mobility is small compared to
the local disease dynamics, it is best to vaccinate the vertex of least degree
and not vaccinate neighboring vertices. Then the epidemic grows on the vertex
corresponding to the largest eigenvalue. When the mobility is comparable to the
local disease dynamics, the most efficient strategy is to vaccinate the whole
network because the disease grows uniformly. However, if only a few vertices
can be vaccinated then which ones do we choose? We answer this question, and
show that it is most efficient to vaccinate along the eigenvector corresponding
to the largest eigenvalue of the Laplacian. We illustrate these general results
on a 7 vertex graph, a grid, and a realistic example of the french rail
network