a heat stabilizer, suspending agent
A translucent brittle solid substance, colorless or slightly yellow, nearly tasteless and odorless, which is created by prolonged boiling of animal skin, connective tissue or bones, usually of bovine or porcine origin, and is one of many types of stabilisers added to vaccines.
Gealtin is responsible for many allergic reactions occurring after vaccination, with symptoms including urticaria, local reactions, and life-threatening anaphylaxis.
Recent studies regarding the safety of gelatin in respect to mad cow disease have prompted the U.S. Food and Drug Administration to re-issue a warning and stricter guidelines for The Sourcing and Processing of Gelatin to Reduce the Potential Risk Posed by Bovine Spongiform Encephalopathy from 1997. Also, in 2004, the U.S. Food and Drug Administration issued a warning about testicular cancer proliferating from gelatin used in Jell-O, because of a certain method used for a short while to process the collagen through molecular reconstruction. Kraft Foods quickly altered their methods.
Wikipedia.com – http://en.wikipedia.org/wiki/Gelatin
Some religious groups who have dietary restrictions for certain animal products are concerned about their presence in vaccines via gelatin. Ingestion of pork products is forbidden in Islam, and some Muslims may avoid medications that contain pork-derived products. Gelatin made from an unclean animals may be impermissible for Jews and Seventh-day Adventists to ingest. Some strict vegetarians or vegans may also choose to avoid animal products vaccines.
Use of bovine-derived products in vaccine manufacture, such as gelatin or bovine sera, has also prompted concern about whether this poses a risk of BSE. BSE is transmitted by prions, which may lead to Creutzfeldt-Jakob disease (vCJD) in humans.
National Library of Medicine: PubChem