CHAPTER 1
How Vaccines Work
What are vaccines? How do they work? These are two seemingly simple questions, but the answers can't be given in a neat sound bite. So, I'm going to ask you to join me for several moments to learn some basic but important information about vaccines.
WHAT ARE VACCINES?
The use of vaccines is based on a simple premise: it is better to prevent a disease than to treat it. Vaccines are substances that prevent disease in the individuals who receive them and protect people who have contact with others who have not been vaccinated.
Basically, vaccines work like this: A weakened or dead form of the organism—typically a bacterium or virus—that causes a specific disease is injected into the body. In the case of combination vaccines, the injection contains weakened forms of more than one organism. The body then produces substances called antibodies to fight the invading organisms, thereby developing immunity against the disease. Those antibodies remain in the body, so if the individual ever has contact with the disease-causing germs the antibodies will be there to destroy them and thus ward off illness.
The main purpose of a vaccine is to stimulate the body to form a high enough concentration of antibodies so that individuals can continue to be protected against the disease. As long as you keep your antibody concentration high (also known as titers, or blood immunity levels), you should have immunity. Vaccinating your child against any specific disease is not a 100 percent guarantee he or she will not get the disease or experience some mild symptoms. However, in most cases, the vaccine provides adequate protection against the disease. That is not to say vaccines do not also cause side effects or complications; they can and do, and this is a topic discussed throughout the book.
HISTORY OF VACCINES IN A NUTSHELL
In the late eighteenth century, Edward Jenner discovered that if he inoculated people with the animal disease called cowpox, people would become immune to the human disease smallpox. Then, in 1885, Louis Pasteur worked on a method called attenuation, which is the use of a weakened form of a virus to provide immunity. As you will see in the following chapters, attenuated vaccines are still widely used today.
Vaccines really burst onto the U.S. scene with the discoveries made by Jonas Salk and Albert Sabin, both of whom developed a vaccine for polio. Salk is credited with an injectable vaccine that was first administered in 1955, while Sabin developed a live oral vaccine in 1961, which was later discontinued in the United States, but an injectable polio vaccine is still used today.
The childhood vaccines parents are familiar with today are the result of years of experimentation, evolution, and development. The first measles vaccine was licensed in 1963, and in the years that followed other childhood vaccines now recommended by the Advisory Committee on Immunization Practices to the Centers of Disease Control and Prevention became part of the current vaccination schedules. And the story is not over: new vaccines, new combinations and formulations, and new dosing recommendations are being announced all the time.
AN INFANT'S IMMUNE SYSTEM
An infant's immune system does not kick into gear until the child is about 6 months old. During those first six months, children depend on the antibodies they received from their mother through the placenta during pregnancy. The only antibodies that cross the placenta are immunoglobulin G (IgG), which makes up 75 to 80 percent of all the antibodies in the body. Mothers who breast-feed their infants continue to pass these essential antibodies plus others via breast milk, which contains five types of antibodies, immunoglobulin A, D, E, G, and M. These antibodies provide a great deal of protection for infants, but they are not foolproof, and so infants are still vulnerable to numerous infectious diseases, such as whooping cough.
Passive Immunity
The type of immunity infants receive from their mothers is called passive immunity because the mother passes her antibiotics to her child. During the first several months of an infant's life, the levels of the antibodies passed from the mother decline steadily. Fortunately, a healthy baby's immune system will begin to produce its own antibodies by age 2 to 3 months. Production is slow at the beginning, however, and so antibodies are not made at a normal rate until about age 6 months in healthy infants.
Please note that I am talking about "healthy" babies, so infants who have a chronic medical condition or who have an infection or other health issue may not reach a normal rate of antibody production until later. This is a concern you will need to discuss with your health-care provider.
When infants are injected with vaccines, they receive the same antigens or parts of antigens that cause disease. However, the antigens in the vaccines have been either killed or greatly weakened (attenuated) (I discuss these types of vaccine later), which means although they are not potent enough to cause symptoms or the disease, they are strong enough to stimulate production of antibodies. Therefore, when children are vaccinated they can develop immunity without having to experience the actual diseases the vaccines are designed to prevent.
Another Type of Passive Immunity
Another type of passive immunity is called herd immunity. This means that an individual who has not been vaccinated at all or has been only partially vaccinated (e.g., has received only part of a necessary series of doses to achieve full immunity) may be protected against getting the disease if all or most of the people around him or her have been vaccinated. That is, the "herd" provides some protection for individuals who have not developed immunity.
Occasions when herd immunity is important include when a very young infant (younger than 6 or 8 weeks old) may be in close contact with other children or adults who have not been vaccinated and when a child who has not been vaccinated attends school with other children who have received their vaccinations. (Read more about herd immunity in chapter 2.)
TYPES OF VACCINES
I have already mentioned the two basic forms of vaccines: killed (also known as inactivated or dead) and live. There is a third type, recombinant DNA vaccine, which is the newest member of the vaccine family and the result of genetic engineering. Here are the basics on each of the three types of vaccines.
Killed Vaccines
To make a killed vaccine, the disease-causing organisms are deactivated, which means, unlike live vaccines, they cannot reproduce nor can they cause the disease they were created to prevent. Because the organisms have been killed, they trigger a weaker response by the immune system than do live vaccines. For this reason they also tend to be safer than live vaccines for certain populations, including pregnant women, children younger than 12 months, and anyone who has a compromised immune system.
As you will learn when you read about different killed vaccines, which are used for hepatitis A, hepatitis B, flu, pertussis, polio (injected), rabies, and typhoid, these vaccines are protein based, like the bacteria they mimic. Some of these bacteria have sugars called polysaccharides on their surface. When scientists created pure polysaccharide vaccines, they discovered these vaccine were not effective in infants. However, when the scientists combined (conjugated) the polysaccharide to a protein they were able to develop vaccines that were effective for infants and young children.
Another type of killed vaccine is a toxoid, which is made when scientists deactivate the poisons (toxins) that are produced by bacteria and viruses responsible for certain diseases. The vaccines to help prevent diphtheria and tetanus are toxoids. You can read more details about toxoids in chapter 5.
Live Vaccines
To create a live vaccine, scientists use the living microorganisms (usually viruses) that cause the disease in question. The viruses are attenuated, or weakened, so they will not cause the disease, but at the same time they will stimulate the body's immune system to create an immune response. Examples of live attenuated viral vaccines include those for measles, mumps, chicken pox (varicella), rubella, and yellow fever. Live bacterial vaccines include one for typhoid fever and one for tuberculosis (Bacillus Calmette-Guérin vaccine).
Recombinant DNA Vaccines
A third type of vaccine is a recombinant DNA vaccine, which is genetically engineered. To make a recombinant vaccine, scientists take specific genes from the infectious agent (e.g., virus, bacteria) and add them to a culture medium, such as yeast or a special broth. The hepatitis B vaccine is an example of a recombinant DNA vaccine, because it is made by culturing a part of the hepatitis B virus gene—the hepatitis B surface antigen (HBsAg)—in baker's yeast.
Copyright © 2012 by Lynn Sonberg
