Vaccines may be categorized as whole-pathogen vaccines, subunit vaccines, or nucleic acid vaccines.7 Whole-pathogen vaccines consist of either live attenuated or inactivated microorganisms. Live attenuated vaccines use a weakened form of the pathogen, which after administration replicates and induces an immune response.
Subunit vaccines are prepared from fractional antigenic components of an organism and may be protein-based, pure polysaccharide-based, conjugate polysaccharide-based, or recombinant protein-based.7 Subunit vaccines typically incorporate adjuvants (e.g., alum, AS04, AS01B, others) to generate a stronger immune response because subunit vaccines alone are not effective in inducing adequate long-term immunity.8
Nucleic acid vaccines (e.g., mRNA vaccines, DNA plasmid vaccines, recombinant vector vaccines) introduce the specific genetic material encoding the target antigen or antigens against which an immune response is sought.7 Once introduced, the body’s own cells then use the genetic material to produce the antigens. Potential advantages of this approach include no risk of infection; improved vaccine stability; stimulation of broad long-term immunity; and rapid, inexpensive, and scalable vaccine manufacturing.9
All vaccines initiate immune responses mediated by macrophages and lymphocytes, i.e., they induce active immunity. Antibodies are produced against specific infective agents or their toxins. Protective antibodies, which may be produced in response to vaccines and their mechanisms of action, are listed in Box 1.10
|Antitoxins||Inactivate toxic microbial protein products|
|Opsonins||Facilitate phagocytosis of microorganisms|
|Lysins||Damage microbial cell membrane|
|Antiadhesins||Prevent adhesion of microorganisms to host cell components|
|Neutralizing antibodies||Prevent the proliferation of microorganisms|
Neutralizing antibodies cannot readily diffuse into infected cells; they must interact with the target-organism before initial intracellular penetration. However, sensitized lymphocytes acting alone or antibodies interacting with lymphoid K-cells may recognize surface changes in infected cells and target them for destruction. Immunization with live attenuated organisms is more predictable than immunization with inactivated microorganisms in inducing long-term immunity.7
Immunogenic responses to live attenuated and inactivated microorganisms develop more slowly than the incubation period of most pathogens. Therefore, vaccines must be administered prior to exposure to a specific etiologic agent. By contrast, "booster" re-immunization in a previously immune individual provides a rapid secondary (anamnestic) increase in immunity. The persistence of immunity may also be prompted by natural re-exposure to microorganisms and the presence of latent infections.