"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, such as the "hygiene hypothesis." 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.
However, adds Dr. Offit, the hygiene hypothesis does not fit vaccine-related diseases. Vaccines do not prevent most common childhood infections, such as upper and lower respiratory tract infections, that form the basis of the hygiene hypothesis. On the other hand, vaccine-preventable infectious diseases such as measles, mumps and whooping cough are easily transmitted regardless of home hygiene. "The flaws in using this biological mechanism to explain a link between vaccines and allergies are consistent with large-scale epidemiological studies," said Dr. Offit. "Those studies found no evidence that vaccines increase the risk of asthma, food allergies or other allergic disorders."
Another set of hypotheses proposes that vaccines cause autoimmune diseases such as 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."
About the Expert:
Paul A. Offit, M.D., is the director of the Vaccine Education Center and chief of Infectious Diseases at The Children's Hospital of Philadelphia. An internationally recognized expert in virology, immunology and vaccine safety, he is a member of the Advisory Committee on Immunization Practices to the Centers for Disease Control and Prevention. In addition to publishing more than 90 peer-reviewed scientific papers, Dr. Offit is co-author of the book "Vaccines: What Every Parent Should Know." He frequently lectures to national and international healthcare organizations about vaccine safety and efficacy.
Under the direction of Dr. Offit, The Children's Hospital of Philadelphia established The Vaccine Education Center in October 2000 to respond to the rapidly growing need for accurate, up-to-date, science-based information about vaccines and the diseases they prevent. The Center is a nationally recognized educational resource for healthcare professionals and parents, providing information on the full spectrum of vaccine-related topics. Approximately 400 people per day visit the Center's comprehensive Web site (vaccine.chop.edu).
Founded in 1855 as the nation's first pediatric hospital, The Children's Hospital of Philadelphia is ranked today as the best pediatric hospital in the nation by a comprehensive Child magazine survey. Through its long-standing commitment to providing exceptional patient care, training new generations of pediatric healthcare professionals and pioneering major research initiatives, Children's Hospital has fostered many discoveries that have benefited children worldwide. Its pediatric research program is among the largest in the country, ranking second in National Institutes of Health funding. In addition, its unique family-centered care and public service programs have brought the 381-bed hospital recognition as a leading advocate for children and adolescents from before birth through age 19. Children's Hospital operates the largest pediatric healthcare system in the U.S. with more than 40 locations in Pennsylvania, New Jersey and Delaware.