"Anecdotal reports and uncontrolled studies have proposed that vaccines may cause particular allergic or autoimmune diseases," says Dr. Offit. "Such reports have led some parents to delay or withhold vaccinations for their children. This is very unfortunate, because the best available scientific evidence does not support the idea that vaccines cause chronic diseases. Scientific studies have shown, however, that reducing vaccination rates lead to increases in preventable infectious diseases."

In the article, co-authored by Charles J. Hackett, Ph.D., of the National Institutes of Allergy and Infectious Diseases, Dr. Offit critically analyzes proposed explanations for a link between vaccines and chronic diseases.

One set of hypotheses proposes that vaccines cause autoimmune diseases such as rheumatoid arthritis, multiple sclerosis or type 1 diabetes by inadvertently stimulating the immune system to attack itself. The mechanism of "molecular mimicry" is based on the fact that some proteins on invading microbes are similar to human proteins. In responding to proteins from the infectious agent, the immune system may mistakenly attack similar proteins in the patient's body, and set off a disease.

Molecular mimicry may indeed allow a natural infection to trigger an autoimmune disease, as when Lyme disease leads to chronic arthritis. However, says Dr. Offit, this process cannot be extended to what happens with vaccines. Naturally occurring viruses and bacteria are much better adapted to growing in humans than vaccines, and are much more likely to stimulate potentially damaging autoimmune reactions.

"Vaccines are engineered to carry weakened or deactivated pathogens, and consequently there are critical differences between natural infection and immunization," said Dr. Offit. "These differences are reflected in the many well-controlled epidemiological studies that do not show a causal relationship between vaccines and autoimmune diseases, including multiple sclerosis, type 1 diabetes and chronic arthritis."

Another set of hypotheses, such as the "hygiene hypothesis," states that improved hygiene and decreased early exposure to common childhood infections may actually raise a child's risk of developing allergies. Several studies support this hypothesis, says Dr. Offit, such as findings that children who attend childcare or live in large families are less likely to have allergies.