PHILADELPHIA -- Large scientific studies do not support claims that vaccines may cause chronic diseases such as asthma, multiple sclerosis, chronic arthritis and diabetes, according to a report in the March 2003 issue of Pediatrics. The report's lead author, Paul A. Offit, MD, chief of infectious diseases and director of the Vaccine Education Center at The Children's Hospital of Philadelphia, identifies flaws in proposed biological explanations for how vaccines cause chronic diseases and reviews current research on associations between vaccines and those diseases.
"Anecdotal reports and uncontrolled studies have proposed that vaccines may cause particular allergic or autoimmune diseases," says 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, PhD, of the National Institutes of Allergy and Infectious Diseases, 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 Offit, such as findings that children who attend childcare or live in large families are less likely to have allergies.
However, adds 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 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 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 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."
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 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.
Source: The Children's Hospital of Philadelphia