The Centers for Disease Control and Prevention, abbreviated CDC, describes the seasonal flu shot as an "inactivated vaccine." This simply means that the shot itself contains a killed version of the influenza, or flu, virus. Specifically, each year, the seasonal flu shot contains the three types of influenza virus that are predicted to be the most common during the flu season. While it is generally a very safe vaccine, there are some potential risks from the flu shot.
Severe Allergic Reaction
While allergic reactions to the flu vaccine are rare, they do occur. Symptoms of a reaction may include wheezing, hives, weakness, dizziness, or difficulty breathing. While an allergic reaction could occur in response to any component of the vaccine--some contain formaldehyde, gelatin, or MSG--the component that is more likely to cause an allergic reaction is the egg component of the vaccine. The Institute for Vaccine Safety, at the John Hopkins Bloomberg School of Public Health, reports that nearly all of the available flu shot formulations may contain residual amounts of egg proteins in the vaccine. This is because the flu viruses put into the vaccine are actually grown in chicken eggs. Patients with an egg allergy are at a higher risk for having an allergic reaction to the shot; anyone who has had a severe reaction to eggs should consult with a physician before getting the flu shot.
Mild Reactions
More common reactions to the shot should also be considered as minor risks of getting the flu shot. Redness at the site of the injection, swelling, and some pain are all possible effects of the shot. Because the flu shot is injected directly into the arm muscle, arm soreness is also extremely common--a study published in the journal Archives of Internal Medicine found that nearly two-thirds of the 424 subjects receiving the flu shot--64% of the subjects--complained of mild arm soreness; in rare cases, people actually complained of a temporary decrease in their ability to comfortably use the arm. Finally, a low-grade fever may occur in response to the vaccine.
Guillain-Barré Syndrome
The CDC reports that Guillain-Barré syndrome, which is a neurological disease that can cause muscle weakness and even paralysis or difficulty breathing, normally occurs in one out of 100,000 people each year. Dr. Patricia Hibberd, of the Massachusetts General Hospital for Children, writing in the medical reference UpToDate, reports that a slightly increased risk for Guillain-Barré syndrome has been associated with the flu shot during certain influenza seasons. This link is based on studies done in the 1970s and 1990s; the CDC estimates that the risk of Guillan-Barré may affect one out of every 1 million people given the flu vaccine.
References
- Questions and Answers: Seasonal Flu Shot
- Components of Seasonal Influenza Vaccines 2009/10
- "UpToDate"; Patricia Hibbard; 2010
- "Archives of Internal Medicine"; Side effects associated with influenza vaccination in healthy working adults. A randomized, placebo-controlled trial; K.L. Nichol et. al.; July 1996
