Immunization against viral diseases.

This chapter provides a review of the current viral vaccine practices and concepts, and indicates future directions for vaccine research and development. ‘‘Active immunization’’ to a virus can be induced through natural infection or by vaccination. ‘‘Passive immunization’’ refers to a transfer of temporary immunity to the host, and provides transient immune-mediated protection from infection or disease. Immunization can influence antibody-mediated immunity in many ways. First, by increasing the repertoire and the frequency of immunoglobulin receptors on memory B cells capable of recognizing a particular antigen, the kinetics of antibody production on subsequent exposure will be more rapid. Second, somatic hypermutation and B-cell selection result in antibodies of successively higher affinities, so repeated virus infection or immunization will cause affinity maturation. Third, immunization with a vaccine formulation that promotes IFN-γ production will increase the production of the immunoglobulin G1 (IgG1) and IgG3 subclasses. Finally, repeated immunization can improve the breadth of antibody responses to a particular virus by recruiting new responses to different antigenic regions. Many killed or inactivated and subunit viral vaccines have also been licensed for the prevention of viral diseases. The major advantage of inactivated vaccines is that there is no risk of infection in the vaccinated patient or of transmission of live virus to potentially immunocompromised close contacts of vaccinees. In general, the most effective way of protecting people with immunodeficiencies from vaccine-preventable diseases is to make sure the people around them are well vaccinated and are not transmitters of infection.