It Increases the chance of a serious adverse reaction. It is a condition in the recipient of the vaccine, not with the vaccine per se. If the vaccine were given in the presence of that condition, the resulting adverse reaction could seriously harm the recipient.

For instance, administering influenza vaccine to a person with a true anaphylactic allergy to egg could cause serious illness or death in the recipient. In general, vaccines should not be administered when a contraindication condition is present.

The most common animal protein allergen is egg protein found in vaccines prepared using embryonated chicken eggs (e.g. yellow fever and influenza vaccines). Ordinarily, a person who can eat eggs or egg products can receive vaccines that contain egg; persons with histories of anaphylactic or anaphylactic-like allergy to eggs or egg proteins should not. Asking persons whether they can eat eggs without adverse effects is a reasonable way to screen for those who might be at risk from receiving yellow fever and influenza vaccines.

True contraindications are very few. Only three permanent contraindications are:

  1. Severe allergic reaction to a vaccine component or following a
    prior dose of a vaccine
  2. Encephalopathy occurring within 7 days of pertussis vaccination
  3. Severe combined immunodeficiency (SCID) as a contraindication
    to rotavirus vaccine (Flowchart 1).

A precaution might increase the chance or severity of a serious adverse reaction, or that might compromise the ability of the vaccine to produce immunity (such as administering measles vaccine to a person with passive immunity to measles from a blood transfusion). Injury could result, but the chance of this happening is less. In general, vaccines are deferred when a precaution condition is present (Flowchart 2).

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