  Infection can spread easily on networks with heterogeneous degree
distribution. Here, we considered targeted immunization on such networks,
wherein a fraction of individuals with the highest connectivity are immunized.
To quantify the effect of this targeted immunization approach on population
immunity, we proposed a method using the type reproduction number.
Consequently, we derived a precise and simple formula that can yield the
immunization threshold, which, to the best of our knowledge, is the first such
result presented in the literature.
