Live Vaccines contain agents capable of replicating within the horse yet have attenuated pathogenicity. Live vaccines stimulate a broad range of immune responses and generally long lasting duration of immunity with the administration of fewer doses. Live vaccines have the potential to induce cytotoxic T-lymphocytes (CTL), or mucosal immunity if administered at mucosal sites, both of which can be very advantageous.

There is potential risk in vaccinating animals whose immune status may be compromised due to disease (i.e. immunodeficiency, hyperadrenocorticism), physiologic states (pregnancy) or medications (i.e. corticosteroids).

Modified Live Vaccines (MLV) are typically derived from the naturally occurring pathogen, and are produced by: 1) attenuation in cell culture, 2) use of variants from other species, and 3) development of temperature-sensitive mutants.

Recombinant Vaccines: 

• Live Attenuated Vector Vaccines are engineered by incorporation of a pathogen’s antigenic peptides into a harmless carrier virus or bacteria.
• Chimeric Vaccines are produced by substituting genes from the target pathogen for similar genes in a safe, but closely related organism. 
• DNA Vaccines consist of a DNA plasmid encoding a viral gene that can be expressed inside cells of the animal to be immunized.

Inactivated/Killed Vaccines lack pathogenicity and can neither replicate nor spread between hosts. These vaccines typically require multiple doses in the primary vaccinal series and regular boosters. Efficacy of inactivated/killed vaccines is often reliant on the use of potent adjuvants.

• Inactivated/killed pathogen vaccines contain whole pathogens that have been inactivated with agents such as phenol (bacteria) and formalin or beta-proprionolcatone (viruses).
• Protein vaccines include naturally produced components of pathogens. These proteins are typically non-pathogenic and may promote fewer injection site reactions than products containing the entire pathogen.
• Recombinant subunit vaccines contain synthetically produced antigens that have been identified as important in developing immunity to a specific pathogen. Currently no such products are licensed for use in equids.
• Adjuvants function to modulate and amplify the host immune response to the accompanying antigen, and are critical to the success of inactivated vaccines.