PRP-protein conjugate vaccines may prove to be a prototype of a new generation of bacterial vaccines for the prevention of diseases caused by encapsulated bacteria, such as pneumococci, meningococci and Group B streptococci, as well as Hib. These diseases often strike the very young, the very old and individuals with compromised immunity. These vaccines have been "engineered" to maximize immune recognition and response. It is hoped that these vaccines will be adequate to control or possibly eradicate Hib and other invasive bacterial diseases. For high risk populations unable to respond to the vaccines, passive prophylaxis may have a role.