The goal of vaccination is to stimulate an immune response that provides some level of protection from disease. Unfortunately, most vaccines do not achieve complete protection from infection and subsequent disease. Vaccines are expected to reduce the severity of disease in infected animals or limit the frequency of disease in the herd.
Many factors, including nutrition, stresses, and the general health of animals, can influence the effectiveness of vaccination. Vaccines should be administered according to label directions and only to systemically healthy animals. Consult your veterinarian for guidance when designing and implementing a herd vaccination program. Vaccines should not be expected to eliminate all disease problems and should be considered only as a tool to be used with other management strategies to mitigate the occurrence and impacts of infectious diseases.
Types of Vaccines
Figure 1. Most vaccines are administered subcutaneously (SQ, under the skin), which can be performed in a triangle of skin in front of the shoulder or in the axillary (under the arm) region. Intramuscular injections should be given only in the neck muscles, using the same injection site.
Killed vaccines and toxoids consist of killed microorganisms, components of pathogens, or by-products of microorganisms in combination with adjuvants, such as aluminum hydroxide or oil, in order to produce a sufficient immune response. The major advantages of killed vaccines are safety and stability of the product. Disadvantages include the need for multiple doses (booster vaccination) to produce a protective immune response.
Subunit vaccines are a type of killed vaccine that contains only part of the virus or bacteria. These vaccines isolate the most important part of the microorganism needed to produce a proper immune response while eliminating other components of the microorganism.
Autogenous bacterial vaccines (autogenous bacterins) are produced from bacteria isolated from samples collected from diseased animals. The bacteria are cultured, killed, and mixed with adjuvant. These vaccines frequently contain high levels of endotoxin and other by-products found in the culture and should be used with caution. Advantages include the production of custom-made, herd-specific vaccines for which a commercially made vaccine might not be available.
Modified Live Vaccines
Modified live vaccines (MLV) contain a small quantity of virus or bacteria that has been altered so that it no longer is capable of causing clinical disease. It is still capable of causing infection in the animal, which is necessary to produce an immune response, but warrants cautious use in some animals. For example, a MLV should not be administered to a pregnant doe that was never vaccinated prior to pregnancy. The immunity produced by MLV typically is longer in duration and more robust than the immunity produced by killed vaccines. Careful handling, mixing, and storage of MLV is critical. Exposure to high temperatures, sunlight, freezing, soaps, or delayed administration from the time of mixing can result in damage to the vaccine. MLV products, once rehydrated with diluent, should be used within an hour of mixing