What Are Vaccines?
A vaccine is a liquid suspension of killed or attenuated (weakened) microorganisms – typically viruses or bacteria - that is administered to prevent or lessen the risk of infectious disease. “Attenuation” is a process that reduces the virulence of an organism, which means that it decreases the organism’s ability to multiply and cause illness. Attenuation can occur naturally or in the laboratory. Attenuated (modified live) vaccines are made from living organisms that still will replicate in the vaccinated animal and cause an immune reaction, but have lost most or all of their ability to cause disease (virulence). Killed or inactivated vaccines are prepared from infectious microorganisms that are dead. Modified live vaccines typically cause a faster, more effective and longer-lasting immunity than killed vaccines. Some newer vaccines use recombinant DNA technology and genetically engineered components to stimulate an immune response in the vaccinated animal.
Vaccination 101 – How Vaccines Work
It is helpful to have a basic understanding of how the immune system works in order to understand the valuable role of vaccines in companion animals. The immune system regulates the body’s ability to recognize and inactivate or dispose of things that it perceives as foreign and harmful. When a potentially infectious foreign substance, called an “antigen,” enters an animal’s body, the immune system initiates a series of complex chemical and mechanical events. Most antigens are unique proteins that are located on the surface of viral or bacterial microorganisms. The immune system responds to antigens in several ways: it produces antigen-specific antibodies (humoral immunity), and it produces specific cells that actively kill or contain the infectious foreign agents (cell-mediated immunity). Both of these responses involve stem cells from the bone marrow that are transformed into mature cells called “lymphocytes,” which have highly specialized properties and functions in combating disease.
Humoral immunity is all about antibodies. Antibodies are unique protein molecules that target and inactivate the particular antigen that stimulated their production. Antibodies are made by lymphocytes when an animal first comes into contact with an antigen. This can occur naturally or in response to a vaccine. Antibodies are like well-armed soldiers that circulate in the bloodstream, always on alert for their particular antigen. Once an immune response has been mounted to an antigen, the body is conditioned to recognize and defend itself against that infectious agent. When an animal comes into contact with that antigen again, its antigen-specific antibodies are activated. They seek out and attach to the antigen, reducing or eliminating the organism’s ability to cause disease. Antibodies and antigens are like interlocking puzzle pieces – they only fit with their specific counterparts.
Cell-mediated immunity involves different lymphocytes that are sensitized by their first exposure to the antigen. Subsequent exposure to the same antigen triggers these cells to release substances that inactivate the foreign organism. Some of these cells actually attack and kill the infectious viral or bacterial organisms.
Vaccination activates the body's humoral and cell-mediated immune responses, providing varying degrees of protection against the antigen in the vaccine. Unlike natural exposure, however, the duration of protection from vaccines may be limited. Some vaccines provide life-long protection, while others only protect against infection for a limited period of time, requiring periodic booster vaccinations to re-introduce the animal’s immune system to the foreign organism. One-time exposure to many antigens is not enough to trigger long-term immunological memory, requiring a series of vaccinations for full protection. Usually, an animal is not considered fully vaccinated until two weeks after a vaccine series is complete. When to revaccinate, or give booster vaccines, is the subject of much debate among immunology experts. Recommendations about boosters have changed over the years, typically to lengthen the recommended time between boosters, especially for viral vaccines. Vaccination protocols should be customized to individual animals, based on their health and particular risk factors.
Kittens and puppies receive maternal antibodies across the placenta before they are born and in their mother’s milk after they are born, through a process called “passive immunity.” Maternal antibodies are highly concentrated in colostrum, which is a thick, yellowish substance secreted from the mammary glands for a few days or weeks before and after the babies are born (the length of time varies by species). It is essential for newborn cats and dogs to ingest colostrum within the first few hours of birth, because the immature lining of their digestive tracts is only able to absorb the large maternal antibody molecules for somewhere between 8 and 36 hours. Maternal antibodies can interfere with a very young animal’s ability to mount an immune response to a vaccine by binding with the antigen in the vaccine, which is why vaccinating a very young animal is usually pointless. Maternal antibodies in kittens and puppies naturally wane somewhere between 6 and 16 weeks, depending upon the concentration of antibodies in maternal milk when the babies were born. Females who were vaccinated just before being bred pass the highest maternal antibody levels to their offspring. Initial kitten and puppy vaccinations are usually given in a series to be sure that the animals get the full vaccine-induced immunity they need, without interference from maternal antibodies.