Influenza and Yellow Fever vaccines are grown
in egg cultures. It is not unreasonable to be concerned about
the possibility that residual egg protein may induce adverse reactions
in patients with known egg induced anaphylaxis.
Measles Mumps Rubella (MMR) vaccines, however, are different.
They are produced in egg fibroblast cultures, and contain miniscule
quantities of contaminating egg protein. Early reports of systemic
allergic reactions to MMR vaccine in some egg-allergic
children led to advice that the use of the vaccine in egg allergic
children should be undertaken with caution. This led to the establishment
of "high risk" vaccination clinics and recommendations
that egg allergic patients be skin tested with the vaccine before
administration. Those with positive tests either had the vaccine
withheld, or administered in graded doses to "desensitize"
them.
In retrospect, the presence of allergic reactions to a vaccine
in some children with egg allergy is hardly surprising, given
that around 0.5 per cent of children are estimated to develop
egg allergy. Subsequent reports, however (summarized in the New
England Journal of Medicine 1995, 332: 1262-1266), suggest that
even in known egg allergic subjects, the risk of an adverse reaction
to vaccination is less than 1 per cent (only 2/1227 reacted).
In fact, the majority of patients with immediate allergic reactions
to MMR were not allergic to egg.
A potential explanation has been offered by a number of papers
over the last 4 years suggesting that the vast majority of those
suffering from immediate reactions to MMR are allergic to the
gelatin stabilizer, added to some live vaccines to enhance stability.
Not only are these subjects not allergic to egg, but the majority
have had (or go on to develop) allergic reactions to oral gelatin
used in products such as fruit gums (J Allergy Clin Immunol 1996;
98: 1058-61).
Gelatin is classified "generally regarded as safe" for
human consumption by the US FDA & is considered to be relatively
hypoallergenic & non-immunogenic. It has a variety of therapeutic
uses including collagen implants for plastic surgery, gelatin
sponges for surgical haemostasis, colloid solutions (Haemacel),
dissolvable contact lenses, as a stabilizing agent in some vaccines
(such as MMR) & as a binding and coating agent in tablets
and capsules. It is also used in photographic emulsions, glues,
matches, cosmetics, and shampoos. Indeed, contact urticaria has
been reported with some of these products. Gelatin is also a common
constituent of many processed foods including confectionery, food
thickeners, dips, glazes, icing, soups, chilled dairy products
(e.g. yogurt, mayonnaise, mousse, ice-cream, cheeses), in smallgoods
(e.g. sausage coatings, salami, tinned hams, pate) and is sometimes
used to clarify fruit juices and wine.
At this time, there is little evidence to support the widespread
concern that infants with food allergy to egg are at "special
risk" of reactions to vaccines. Nevertheless, any vaccine
at any time may induce anaphylaxis and the availability of resuscitative
equipment is still required.
See also an Editorial
in the BMJ dated 1 April 2000.
