Vaccines are extremely effective, although some are better at preventing diseases than others. If you simply measure the body’s ability to produce antibodies to fight a specific disease, vaccines are very effective, with an over 95 percent success rate for many of the common vaccines. Real-world protection is usually higher because of herd immunity.
This does not mean that vaccines are perfect. Nothing protects 100 percent of the people 100 percent of the time. To use a specific example, only 98 percent of people who receive the normal three-dose series of the hepatitis B vaccine develop antibodies. Some people can get up to nine vaccinations of hepatitis B without any response! In the case of hepatitis B, it appears that a slight genetic abnormality in the immune system prevents the vaccine from working in some individuals.
In addition, as previously mentioned, immunity can fade over time. One example of this involves the chickenpox vaccine. Recent studies have shown that about 15 percent of children who received the vaccine at twelve to fifteen months still were able to contract the illness four to five years later. This is why the national guidelines now recommend a booster dose for chickenpox before children go to kindergarten.