Live, attenuated vaccines
Live vaccines are made from live viruses that have been grown under special laboratory conditions so they are weakened, or attenuated, and thereby do not cause the symptoms or complications of the disease. But they do produce immunity, because the live vaccine virus displays the same special markers on its outer surface as the wild-type virus. When the live vaccine virus enters the body and begins replicating itself, the immune system jumps into action. Each component of the immune system does its part to process and destroy the antigen, and memory cells develop to guard against the next invasion of the disease.
The entire immune system responds to live vaccines as it does to infection from the wild-type disease. Thus, live injected vaccines usually require only one dose for lifelong immunity, just as one bout of measles or chickenpox will make most people immune for life.
Live vaccines, however, have several disadvantages. First, they usually require special storage and handling to keep them alive. Second, they could overwhelm a person who does not have a well-functioning immune system. This is why live vaccines should generally be withheld if a person has a weakened immune system.
Because of concern about the possibility that a live vaccine virus could harm a fetus, these vaccines are not given to pregnant women. Also, because antibodies passed from the mother to the fetus may interfere with live injected vaccines, these live vaccines are usually recommended at 12 months of age or later.
Because live vaccines are weak versions of the disease germs, they may cause a mild case of the disease they were designed to prevent. For example, chickenpox vaccination may cause a person to break out in a few pox or develop a low-grade fever. In real chickenpox a person usually develops 200 pox and substantial fever.
Live vaccines are used for the following diseases:
| Measles | Mumps | Rubella | Chickenpox |
| Polio (Oral) | Typhoid (Oral) | Tuberculosis | |
| Yellow Fever | Smallpox |
Inactivated vaccines
Inactivated vaccines consist of whole microbes that have been killed by heat or chemicals (as in the inactivated polio vaccine, IPV) or are simply the important part of the microbe that provokes the immune system to respond (as in the hepatitis B vaccine). Unlike live vaccines, inactivated vaccines cannot replicate and so cannot cause even mild cases of the disease, but their presence still prompts the immune system to respond.
The inactivated vaccines cause a relatively weak response by the immune system, so usually the vaccination must be repeated. The advantages to inactivated vaccines include that they are not as fragile as live vaccines. Unlike live vaccines, inactivated vaccines are safe for persons who have weakened immune systems, for pregnant women, and for children under a year old. The side effects are generally just soreness where the vaccine was injected and possibly some fever shortly after vaccination.
Usually the outer coat of bacteria is made of protein, and the inactivated vaccines mimic this protein. Protein-based inactivated vaccines are used for the following diseases:
| Influenza | Polio (injected) | Pertussis | Plague |
| Hepatitis B | Hepatitis A | Lyme disease | |
| Rabies | Typhoid | Cholera |
Some bacteria are literally sugarcoated. That is, their outer surface is composed of complex sugars, or polysaccharides . Vaccines against these bacteria must duplicate these special coats. Unfortunately, pure polysaccharide vaccines don’t work well in infants and don’t produce increasingly high antibody levels with subsequent doses. The meningococcal vaccine and the pneumococcal vaccine for older children and adults are pure polysaccharide vaccines.
A superior vaccine is produced by joining (conjugating) the polysaccharide to a protein. Conjugate vaccines are effective in infants and boost antibody levels with subsequent doses. The current Hib and the pneumococcal vaccine for infants and young children are conjugate polysaccharide vaccines.
Toxoids are another kind of inactivated vaccine. Instead of being made from killed germs, toxoids are made by inactivating the poisons, or toxins, produced by the germs. After you take the toxoid, your body is no better at fighting off infection to those germs, but it is much, much better at fighting off the effects of the germ’s poison. The vaccines against tetanus and diphtheria are toxoids.
This information is excerpted from the book Vaccinating Your Child: Questions and Answers for the Concerned Parent (Peachtree Publishers, Ltd., 2000). The book’s authors are Dr. Sharon G. Humiston, a pediatrician and clinical researcher at the CDC and the University of Rochester, and Cynthia Good, an award-winning journalist and host of the television show “Good for Parents”.