On February 8, 2001, the New York Times published an article entitled "Five drug makers use material with possible mad-cow link". This article followed a Public Health Service statement on December 22, 2000 in Morbidity and Mortality Weekly Report (MMWR). MMWR is written by the Centers for Disease Control and Prevention (CDC). The New York Times article and CDC report were prompted by the confluence of several events. First, as of July 2000 about 175,000 cows in the United Kingdom developed a disease called "mad-cow" disease — a progressive disease of the nervous system of cattle. Second, at least 80 people in the United Kingdom developed a progressive neurological disease called variant Creutzfeld-Jakob disease (vCJD) that may have resulted from eating meat prepared from cows with "mad-cow" disease. Third, some vaccines are made with serum or gelatin obtained from cows in England or from countries at risk for "mad-cow" disease.
vCJD is caused by an unusual protein called a prion (proteinaceous infectious particle). Prions are found in the brains of cows with "mad-cow" disease and in the brains of humans with vCJD. Prions can also be found in the spinal cord and in the back of the eye (retina).
However, blood from infected animals or blood from infected people has never been shown to be a source of infection to humans.
The likely source of prions for people in England was hamburger, not steak, prepared from cows. Hamburger may be prepared in a manner that includes the spinal cord. Steak, on the other hand, represents only the muscles of cows and, therefore, does not contain prions.
Viral vaccines are weakened forms of natural viruses. Some viral vaccines are made by "growing" viruses in specialized cells in the laboratory. Many growth factors are needed for cells to grow. An excellent source of these growth factors is serum obtained from the fetuses of cows (known as fetal bovine serum). Fetal bovine serum is a naturally filtered source of growth factors. The natural filter is the bovine placenta. Whereas the human placenta contains 1½ layers that separate the mother's blood from fetal blood, the bovine placenta contains 6 layers. Many proteins are excluded from the bovine fetal circulation by these 6 layers (for example, bovine fetal blood contains 1/500th of the antibodies found in bovine maternal blood).
Another product from animals that may be used in vaccines is gelatin (see Do vaccines contain gelatin?). Gelatin is a protein formed by boiling skin or connective tissue. Gelatin is used to stabilize vaccines so that they remain effective after manufacture.
To answer this question, let's go through each step of the manufacturing process:
When you put all these factors together, the chance that currently licensed vaccines contain prions is zero.
The Public Health Service is interested in maintaining the public's trust in immunizations. They are concerned that the public may fear that vaccines containing bovine material from countries at risk for "mad-cow" disease could potentially transmit this disease to children. So they have taken the precautionary step of eliminating the use of these materials in the production of vaccines.
However, the facts about prion transmission should reassure us that it is essentially impossible for currently licensed vaccines to contain prions.
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Reviewed by: Paul A. Offit, MD
Date: April 2013
Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.