By July of 2000 approximately 175,000 cows in the United Kingdom had developed bovine spongiform encephalopathy, more commonly referred to as MCD, a progressive deterioration of the nervous system. At the same time, >70 people in the United Kingdom had developed a progressive neurologic condition termed variant Creutzfeldt-Jakob disease (vCJD) that likely resulted from eating meat prepared from cows with MCD. Both MCD and vCJD are caused by prions, which are proteinaceous, self-replicating infectious particles.
In July 2000, the FDA convened a meeting to discuss the possibility that some vaccines were made using serum or gelatin derived from cows in countries that had MCD, including England. Although the risk of transmission of vCJD to humans from such vaccines was considered theoretic and remote, the recommendation was made that vaccines use bovine materials originating from countries without endogenous MCD. Mathematical models suggest that the agent of MCD first entered cattle feed in the United Kingdom around 1980; since the vast majority of initial cases of vCJD were born well before then, childhood vaccines were not likely to be the cause.
Prions are found in the brains of cows with MCD and in the brains of humans with vCJD. They can also be found in the spinal cord and retina. However, blood from infected animals and from infected people has never been shown to be a source of infection of humans. The likely source of prions for people in England was hamburger, not steak, and 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.
Vaccines may contain trace amounts of animal products used during the manufacturing process. For example, vaccines are grown in laboratory cells that require growth factors for maintenance. An excellent source of these is fetal bovine serum, which is naturally filtered by the 6-layered bovine placenta. Many proteins are excluded from the bovine fetal circulation by these layers (for example, bovine fetal blood contains 1/500th of the antibodies found in bovine maternal blood). Maternal-fetal transmission of prions has never been documented in animals and fetal blood is not known to contain prions. Moreover, the fetal bovine serum used in vaccine manufacture is highly diluted and eventually removed from cells during purification of vaccine viruses. It should be pointed out as well that prions propagate in mammalian brain but not in cell culture.
Another product from cows and pigs that may be used in vaccines is gelatin, a protein formed by boiling skin or connective tissue such as hooves. Gelatin is used to stabilize vaccines so that they remain effective after distribution. Since prions are not detected in the skin or connective tissue of animals, gelatin does not represent a risk of transmission to patients.
Final reassurance comes from the fact that transmission of prions occurs from eating the brains of infected animals or from directly inoculating preparations of brains of infected animals into the brains of experimental animals. Transmission of prions has not been documented after inoculation into the muscles or under the skin, which are the routes used for vaccination. Taken together, the chances that currently licensed vaccines contain prions and represent a risk to humans is essentially zero.
—Marshall, Gary S. “Addressing Concerns About Vaccines.” The Vaccine Handbook: A Practical Guide for Clinicians. 3rd ed. New York: Professional Communications, Inc., 2010. 233-234. Print.