Researchers found that out of a small number of people believed to have suffered serious allergic reactions to the MMR vaccine, about one quarter showed hypersensitivity to gelatin. Gelatin is used as a heat stabilizer in the manufacture of a number of vaccines, the study authors note in the December online issue of the journal Pediatrics.

In most cases, reactions to MMR vaccination are fairly mild and include a rash or fever. Rarely, serious allergic reactions marked by symptoms such as breathing problems, hives and rapid heartbeat occur. Immediate, severe reactions have sometimes been attributed to egg allergy because the MMR vaccine is cultured (grown) in chick embryo cells and may contain bits of egg proteins. But research has indicated that most MMR reactions are not associated with egg allergies.

Gelatin, on the other hand, has been implicated in vaccine reactions in recent years. The US Centers for Disease Control and Prevention (CDC) advises that anyone who has ever had a life-threatening allergic reaction to gelatin not get the MMR.

In the new study, a team led by Dr. Vitali Pool of the CDC in Atlanta, Georgia looked at data from a national registry of adverse vaccine reactions. Between 1991 and 1997, there were 168 "probable" or "possible" serious allergic reactions to MMR vaccination. The investigators were able to get blood samples from 22 of these people, who ranged in age from 15 months to 33 years. They then compared the samples with blood taken from 27 individuals with no history of serious MMR reaction.

They found that 27% of those with MMR reactions had antibodies to gelatin in their blood, indicating they were allergic. No one in the other group had these antibodies, according to the report.

"Results from this study support the hypothesis that (allergic reactions) after MMR vaccines can in some cases be attributable to hypersensitivity to gelatin," Pool's team writes.

They advise that anyone with a history of severe reaction to a gelatin-containing vaccine be tested for antibodies to gelatin and other potential allergens before getting another dose of any gelatin-containing vaccine.

As for the roughly three quarters of MMR-reaction patients who did not show sensitivity to gelatin, the cause of their reactions remains unclear, according to the researchers. They call for more research to unearth other causes of allergic reactions to vaccination.

SOURCE: Pediatrics 2002;110:e71.