Limitations and Possible Side Effects of Virus Vaccines

The purpose of vaccination is to induce immunity to specific diseases without harming the vaccinated animals. This would seem a reasonable purpose and one that could easily be done. Unfortunately, this is not always simple. Problems arise both in establishing strong, longlasting immunity and in avoiding harm associated with vaccination. A brief simplified review of the nature of the immune response of cattle is presented to provide background information to better understand the limitations and problems associated with immunization. The substances directly involved in immunization are antigens (the viruses) and antibodies (the neutralizing sustance). An antigen is usually protein in nature and is not normally present in the animal. Under the right conditions the immune system of the animal will recognize this antigen or virus as foreign or different from the animal's own proteins. An antibody is a protein produced by the animal which will attach to a specific virus or antigen and prevent it from infecting any cells of the animal. This attachment ability is due to the specific physical-chemical structure of the antibody that allows it to be of the right configuration or molecular shape to match the surface of the antigen. The prevention of virus infection is called virus neutralization. Certain tissue cells have as their main function the recognition of foreign substances and the subsequent stimulation of other cells to produce antibodies to neutralize these. Other ceils will devour the neutralized antigens or viruses, to complete the eradication. There are many kinds of antibodies which can function in other ways, but we shall consider only vii"asneutralizing antibodies and their action. A certain mass of virus must be present to induce any antibody production. Once this amount is available, it will be recognized as foreign, and virus-neutralizing antibodies will be produced. Viruses must infect cells to replicate, since they cannot grow outside of a living cell. Although it t~kes a minimal amount of antigenic mass to induce antibody production, it requires a substantially greater amount to cause antibody production to continue or persist for months or years. When the antigenic material that induces antibody production disappears, production will soon stop unti l more antigens appear. When a killed virus is used in a vaccine, it will stimulate antibody production which will last only until the killed virus disappears. Since the killed virus cannot infect cells and replicate, it must be inoculated in large amounts to begin with. Then it must either be taken up by an antigen-retaining cell which apparently exists or must be mixed with a preparation that will only slowly release the dead virus so that it will not disappear rapidly and will stimulate antibody production over a longer time. On the other hand, ff the virus for the vaccine is alive, it can infect the animal's cells and replicate to provide a large amount of antigenic material. I n addition, live viruses can persist within cells where antibodies cannot reach them. Most viruses will remain in a few cells in the animal and occasionally be released to keep the immune system stimulated.