What Are Vaccines?
Vaccines are liquid suspensions of dead or weakened organisms – usually viruses or bacteria - that reduce the risk of infection by those organisms. Several types of vaccines are available for dogs: modified live (attenuated) vaccines, killed (inactivated) vaccines, and recombinant vaccines.
- Modified Live Vaccines. Attenuation is a process that reduces an infectious organism’s ability to make an animal sick. This modification can occur naturally or in the laboratory. Modified live vaccines are made from live bacteria or viruses that trigger an immune reaction in the vaccinated dog, but have lost most or all of their ability to cause infectious disease.
- Killed Vaccines. Killed vaccines are made from dead organisms, which can’t cause infection but can stimulate an immune response in the vaccinated animal. Modified live vaccines typically cause a faster, more effective and longer-lasting immunity than killed vaccines.
- Recombinant Vaccines. Some newer vaccines use recombinant technology and genetic engineering to alter potentially infectious organisms. These include live-vectored, subunit and DNA vaccines.
How Do Vaccines Work?
To understand vaccines, it helps to have a basic understanding of how the immune system works. Basically, the immune system is in charge of protecting the body from things that it perceives as foreign and harmful. Bacteria and viruses have special molecules on their surface called “antigens.” When an infectious organism enters a dog’s body, the immune system makes antibodies (humoral immunity), and makes specific cells that find and destroy the foreign organisms (cell-mediated immunity). Both responses rely on stem cells from the bone marrow that mature into cells called “lymphocytes.” Lymphocytes specialize in fighting disease.
- Humoral Immunity. Antibodies are special molecules that target and inactivate infectious organisms. They are made the first time that viruses or bacteria enter a dog’s bloodstream and are the mirror-image of the antigen molecules on the organism’s surface. Antibodies are primed to recognize the particular infectious agent. When the same virus or bacteria gets into a dog’s bloodstream again, the antibodies physically attach to its antigens, preventing or reducing its ability to reproduce and cause disease. Antibodies and antigens are like interlocking puzzle pieces – they only fit with their specific counterparts.
- Cell-Mediated Immunity. This immune response relies on special cells, called lymphocytes, which are made the first time a dog is exposed to infectious viruses or bacteria. The next time the dog is exposed, these lymphocytes release specific substances that inactivate or kill the organisms.
How Vaccines Work
Vaccines stimulate immunity by introducing killed or modified infectious agents (along with their surface antigens) into an animal’s bloodstream. Some vaccines provide life-long protection, while others protect for a limited period of time. Because one exposure to an antigen might not trigger long-term immunological memory, many vaccines are given in a series. A dog is considered fully vaccinated two weeks after a vaccine series is completed. Most vaccines need to be boosted periodically to re-prime the immune system. Whether and when to revaccinate are the subject of much debate.
Maternal Antibodies. Puppies get antibodies from their mother across the placenta, and later in milk. This creates “passive immunity.” Females vaccinated 2 to 4 weeks before being bred pass the most maternal antibodies to their offspring. Maternal antibodies are concentrated in the first part of mothers’ milk, called colostrum. Newborns need colostrum as soon as possible, because the immature lining of their digestive tracts can only absorb large maternal antibody molecules for a short time. Vaccinating a very young puppy is pointless, because maternal antibodies will bind to antigens in the vaccine and prevent a normal immune response. Maternal antibodies wane somewhere between 6 and 14 weeks of age.