Most people who receive vaccines will experience no allergic reaction to the medication. Allergic reactions are rare for virtually all vaccines. However, certain ingredients commonly found in vaccines may trigger symptoms in those who have allergies to the specific ingredient.

Gelatin is frequently a source of allergic reactions to vaccines. It is used as a stabilizer in vaccines. Those with gelatin allergy have the potential to react when being immunized with any of the following vaccines that contain gelatin:

• Diphtheria, tetanus and pertussis (DTaP)
• Influenza
• Japanese encephalitis
• Measles, mumps and rubella (MMR)
• Rabies
• Varicella
• Yellow fever

Gelatin is not the only allergic trigger associated with vaccines. Egg protein is also a common source of allergic reaction to vaccines. While allergens are often contained within the vaccine, individuals with latex allergies may react to vaccines supplied in rubber-containing vials or syringes.

Vaccines that have caused allergic reactions in some people include:

• Hepatitis A (infectious hepatitis). This vaccine rarely causes reactions in patients, but mild to severe allergy symptoms may occur.
  
  
• Hepatitis B (serum hepatitis). Some patients have experienced anaphylactic episodes when receiving this vaccine. Some experts warn that those with an allergy to baker’s yeast should not get this immunization, as the vaccine contains baker’s yeast. However, other studies say there has never been a substantiated claim of a vaccine-related reaction to this yeast.
  
  
• Measles, mumps and rubella (MMR) vaccine. A very small percentage of children may have an anaphylactic response, which is likely a reaction to the gelatin used as a stabilizer in the vaccine. However, most experts say this tiny risk pales in comparison to the potential benefits from the vaccine. Each of the three diseases can cause serious consequences, including death (measles), brain infection (measles and mumps), and birth defects and mental retardation (rubella).
  
  For years, it was believed that children with egg allergies were highly susceptible to reacting to MMR vaccine. In preparing measles vaccine, the virus must be incubated in cell cultures that contain chicken cells derived from the egg. However, recent studies have found no connection between egg allergy and reaction to the MMR vaccine. Parents are urged to consult with a physician about the source of the vaccine and whether or not to vaccinate children who are allergic to eggs. Some physicians may choose to perform a skin test on the child prior to administering the vaccine.
  
  
• Diphtheria, tetanus and pertussis (DTaP) vaccine. Generalized reactions to the vaccine are rare; however some patients develop IgE antibodies to this vaccine but usually receive subsequent doses without an allergic reaction.
  
  
• Smallpox vaccine. Those with atopic dermatitis or with a history of the condition should not be vaccinated for smallpox, according to U.S. Centers for Disease Control (CDC) guidelines. The only exception to this rule would be during an outbreak of smallpox. In such cases, the potential protective benefit may be worth the risk of side effects. The most vulnerable patients include:
  
  
• • Patients with atopic dermatitis, or a history of the condition.
    
    
  • Patients with congenital or acquired immune deficiency diseases such as AIDS.
    
    
  • Patients taking immunosuppressive drugs. These drugs are often taken by those receiving organ transplantation.
    
    
• Influenza vaccine. Severe allergic reactions to this vaccine are rare. Those with egg allergies or a previous reaction to this vaccine should not be immunized with a yearly flu shot. This vaccine is grown in chicken eggs. Although almost all of the egg protein is filtered out of the final product, some of the allergen may remain. One study done in the late 1980s found that up to one-third of those who receive influenza vaccine showed an increase in IgE antibodies to egg protein when tested.
  
  
• Haemophilus influenzae Type B vaccine.  Also known as Hib disease, Haemophilus influenzae Type B is a bacterial illness that can cause a potentially fatal brain infection in young children. Reports of reactions to this vaccine have been rare.
  
  
• Pneumococcal vaccine. This vaccine can help prevent the lung infection pneumonia, which is caused by pneumococcus bacterium. Allergic reactions are rare, but can occur in some patients.
  
  
• Meningococcal vaccine. Protects against meningococcal disease, a bacterial infection that can lead to bacterial meningitis. This potentially life-threatening disease involves an inflammation of the membrane that covers the brain and spinal cord. Serious allergic reactions to this vaccine are rare.
  
  
• Yellow fever vaccine. People with egg allergies have experienced anaphylactic reactions, as the vaccine is cultured in chicken embryos and contains egg protein.
  
  
• Japanese encephalitis vaccine. Some patients experience hives and wheezing from five minutes to an hour after receiving the vaccine. In addition, this vaccine sometimes can trigger a late-onset combination of hives and angioedema. This often takes place two to three days after immunization. Symptoms usually are confined to the skin, but sometimes blood pressure drops (hypotension) and the individual experiences respiratory stress. Some studies have suggested a link between prior history of hives, asthma, rhinitis or eczema and this late-onset condition.
  
  
• Varicella vaccine. This chickenpox vaccine can cause symptoms consistent with anaphylaxis. It is believed that gelatin used as a stabilizer in the vaccine is the likely cause of reactions.