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 Dec. 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.

What causes progressive neurological diseases like "mad-cow" disease or vCJD?

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.

If prions are only found in the brain and spinal cord, why did people in England get vCJD after eating meat from cows?

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.

Why do vaccines contain materials derived from cows?

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 one-and-a-half layers that separate the mother's blood from fetal blood, the bovine placenta contains six layers. Many proteins are excluded from the bovine fetal circulation by these six 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.

Do vaccines that have been exposed to bovine materials during manufacture pose a risk for transmission of vCJD?

The chance that currently licensed vaccines contain prions is zero. To explain why, let's go through each step of the manufacturing process:

• Cows with "mad-cow" disease have prions in their brain, spinal cord and retina. However, prions are not detected in their blood, skin or connective tissue.
• Fetal bovine serum is used in the manufacture of vaccines. Fetal bovine serum is obtained from fetal blood, and blood is not a source of infection with prions. In addition, although cows "share" their blood with their unborn calves, the bovine placenta is a natural filter. Maternal-fetal transmission of prions has never been documented in animals.
• Fetal bovine serum is highly diluted and eventually removed from cells during the growth of vaccine viruses.
• Prions are propagated in mammalian brains and not in cell culture used to make vaccines. Therefore, prions are unlikely to be propagated in the cells used to grow vaccine viruses.
• Gelatin is also used in the manufacture of vaccines. Gelatin is added to vaccines at the end of the manufacturing process. However, gelatin is made from materials that do not contain prions (skin and connective tissue). In addition, the preparation of gelatin often includes heat sterilization or treatment with organic solvents. It is likely that these treatments would inactivate prions.
• Transmission of prions occurs from either eating brains from infected animals or, in experimental studies, from directly inoculating preparations of brains from infected animals into the brains of experimental animals. Transmission of prions has not been documented after inoculation into the muscles or under the skin (routes used to vaccinate).

If vaccines pose no risk for progressive neurological diseases, why did the Public Health Service choose to eliminate bovine-derived materials obtained from countries at risk for "mad-cow" disease?

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.

References

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