Vaccine Safety

Dr. Offit's Testimony

This is Dr. Paul Offit's testimony to the House Government Reform Committee, given on 6 April 2000. Dr. Offit addressed the question of: Does the MMR vaccine cause autism?

My name is Paul Offit. I am a pediatrician. I am also the Chief of Infectious Diseases and the Henle Professor of Immunologic and Infectious Diseases at The Children’s Hospital of Philadelphia and the University of Pennsylvania School of Medicine. I am also a member of the Advisory Committee on Immunization Practices to the CDC. I have 20 years of experience in the areas of virology and immunology.

My role, in these proceedings, is to explore the theories that have arisen due to concerns by the public that autism might be caused by the combination of measles, mumps, and rubella vaccines (known as MMR). No evidence exists that proves this association. However, three theories have been used to explain it. In the time that I have been given, I would like to explain why I think that these theories are not valid.

The first invalid theory is that children who get the measles vaccine make an immune response not only to the vaccine, but also to their own nervous system. This kind of reaction is called autoimmunity. To understand why this theory is invalid, we must first understand differences between natural measles infection and measles vaccination.

During natural measles infection, the measles virus reproduces itself many times in the body and causes disease. In contrast, following measles vaccination, the vaccine virus reproduces itself much less and doesn’t cause disease. Because more measles proteins are made during natural infection than after immunization, the immune response to natural infection is greater than the immune response to immunization.

If the immune response is greater after natural infection, then the autoimmune response would also be greater. If this were the case, then autoimmunity should occur more frequently after natural infection than after vaccination. Or, said another way, if measles virus caused autism, measles vaccination would lower, not raise, the incidence of autism.

The second invalid theory is that the child’s immune system is simply overwhelmed by seeing three viruses in a vaccine at the same time. Some have gone so far as to suggest that it may be of benefit to divide the MMR vaccine into three separate vaccines. The rationale behind this theory is that children do not normally encounter such an assault on the immune system.

From the birth canal and beyond, infants are confronted by a host of different challenges to their immune system. Their intestines encounter foreign proteins in milk and formula. Their lungs encounter bacteria inhaled on the surface of dust in the air. And literally thousands of different bacteria immediately start to live on the skin, as well as on the lining of the nose, throat, and intestines. Here is how infants deal with this immediate confrontation to their immune system.

Babies have a tremendous capacity to respond to their environment from the minute they are born. The newborn has billions of immunologic cells that are capable of responding to millions of different microorganisms. By quickly making an immune response to bacteria that live on the surface of their intestines, babies keep those bacteria from invading their bloodstream and causing serious disease. Therefore, the combination of the three vaccines contained in MMR, or even the 10 vaccines given in the first 2 years of life, is literally a raindrop in the ocean of what infants successfully encounter in their environment every day.

The third invalid theory is that the MMR vaccine is given by an unnatural route. The rationale behind this theory is that children do not normally encounter viruses or bacteria under their skin or in their muscles. But infants and children frequently encounter viruses and bacteria in many places throughout the body. Our species survives because, from the minute we are born, we are capable of meeting challenges at all sites.

To review, here are the medical facts:

  • First, if autism is a consequence of autoimmunity, then the incidence of autism would have decreased, not increased, after vaccination.

  • Second, children from birth are confronted with and manage an enormous array of different challenges to their immune system at the same time.

  • Third, challenges to their immune system occur by a variety of routes.

The parents we’ve heard testifying here today are asking a scientific question, "Does the MMR vaccine cause autism?" Questions of science are best answered by scientific studies. And the answer to this question is already available. Dr. Brent Taylor and his coworkers in London have conducted a large, meticulously designed, well-controlled study that disproved an association between MMR vaccine and autism. I believe other studies will confirm Dr. Taylor’s results.

"What is at stake here?"

In the early 1990s our immunization rates against measles dropped only about 10%. Measles outbreaks swept across our country. About 11,000 people were hospitalized and 123 died from measles – died from a disease that is easily and safely prevented by a vaccine.

My concern, Mr. Chairman, is that some parents listening to or reading about this hearing might incorrectly conclude that vaccines cause autism. This is clearly not the case – vaccines are extremely safe and highly effective. I encourage this committee to make that fact clear to every parent in America.

If, as a result of reading about this hearing, some parents choose to withhold or delay vaccines for their children, their tragedy could be profound. If many parents choose to withhold vaccines, the tragedy all across America could be devastating.

Let’s proceed cautiously, carefully, and scientifically.

Mr. Chairman, I am ready to respond to any questions the committee might have.


Important disclaimer: The information on is for educational purposes only and should not be considered to be medical advice. It is not meant to replace the advice of the physician who cares for your child. All medical advice and information should be considered to be incomplete without a physical exam, which is not possible without a visit to your doctor.

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