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What are vaccine side effects?
The overwhelming majority of vaccine-related side effects are mild and
short lived, and carry significantly less risk for a person than the risk of having severe illness or even death from having the infection that the vaccine is designed to prevent.
One of the difficulties in deciding whether an event is related to the vaccine or not is that many mild side effects are common occurrences that may have occurred
without having a vaccine. For example, if a teenager plays a game of football on the same day a vaccine is received and then complains of muscle pain, it is almost impossible to say which event really caused the side effect. Similarly,
the influenza vaccine recipient who receives the vaccine at a time of the year when flu-like and respiratory viral illness are common
and then develops symptoms in the days after vaccination, can be experiencing a side effect or just have another viral illness.
The Vaccine Adverse Event Reporting System (VAERS) encourages the community to report all adverse events after vaccination in order to form hypotheses and investigate which of these events are actually vaccine-related.
Vaccine side effects may be classified as mild, moderate and severe. Through the numerous safeguards in place during
vaccine pre and post-licensure processes, most vaccine side effects
that occur are mild, short-lived and are easily explained by understanding the science behind
vaccine manufacturing and by
considering how they are administered.
Mild side effects do not generally affect the person’s ability to carry on their normal activities and resolve quickly.
Examples include:
- Redness, mild tenderness or bruising around the injection site
- Nausea
- Mild irritability
- Fatigue
- Mild muscle pain
- Low-grade fever
Some individuals may experience more moderate side effects such as:
- Larger reactions around the injection site
- High fevers sometimes associated with seizures
- Seizures
- Headaches
- Syncope (fainting)-more commonly found in adolescents and young adults
- Prolonged or excessive crying (for example with DTaP)
- Mild "viral-like" syndromes after receiving live virus vaccines
- Temporary lowering of the platelets (for example, with MMR)
- Temporary joint pain or swelling (for example, with MMR)
Serious side effects with licensed vaccines are very rare and are generally caused by allergy to some of the vaccine components
For a full list of side effects associated with individual vaccines please refer to the following list.
What causes vaccine allergies?
Just as drugs and certain foods can cause allergies, any individual can be allergic to a particular vaccine. In most cases, the allergy is caused not by the killed or inactivated virus or bacterium but by some other vaccine component that is needed to stabilize or preserve the vaccine. Allergic reactions vary in severity. In their mildest form they may consist of itching and a skin rash or hives. Anaphylaxis or severe hypersensitivity reaction causing swelling of the throat and low blood pressure are thankfully extremely rare and are treated by the administration of epinephrine and other anti-allergy medication.
Some possible vaccine components that can cause anaphylaxis are listed below:
Gelatin is used as a vaccine stabilizer in some DTaP, influenza, MMR, varicella and rabies vaccines. Case reports and studies of anaphylaxis in response to MMR vaccine have shown that some of these patients have antibodies to gelatin in their blood and some report a history of food allergy to gelatin containing products. Gelatin allergy is the most common identifiable cause of severe vaccine allergy although the incidence remains very low (one case per
2 million doses).
Egg allergies are important because they are relatively common in the population (less
than 1 percent of the population) and are found in one in 20 children with a history of allergies. Some vaccines,
including influenza
and yellow fever, are made in eggs and as a result some egg proteins are present in the final vaccine. These small quantities of egg protein may be sufficient to cause severe reactions (potentially fatal)
those with egg allergies. However, patients at high risk for influenza infection have safely been given vaccine under specially designed protocols. Contrary to the belief of some, MMR vaccine is made in chick embryo cells in culture (not in eggs) and the amount of egg protein in this vaccine is not sufficient to cause severe allergic reactions
Antibiotics can theoretically cause vaccine allergies. Antibiotics are necessary to prevent bacterial contamination during vaccine manufacture but the vaccines used (neomycin, polymixin
B, streptomycin and gentamicin) are far less likely to cause allergies than antibiotics like penicillins, cephalosporins
and sulfa drugs. Of the antibiotics used in vaccine manufacture,
only neomycin is present in the final product, including MMR and rabies vaccines.
A full history of patient allergies is necessary before vaccine administration and any suspected vaccine reaction should be reported to
VAERS. An allergic reaction to a vaccine may preclude further administration of that vaccine (except in exceptional circumstances) and expert advice should be sought.
 
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