If the odds of winning the lottery were 1 in 5, the line for tickets would be long indeed. In fact, as many as 1 in every 5 Americans get the flu each year, according to the Centers for Disease Control and Prevention (CDC). These high odds could be decreased if more Americans took advantage of influenza immunization. The CDC estimates that vaccination can cut flu-related visits to medical facilities by about 60 percent in adults. But worries about other odds -- particularly the odds of experiencing negative side effects from a flu shot -- keep some folks from getting vaccinated even though the benefits generally outweigh the risks.
Most Common Side Effect
Flu shots have been available commercially in the United States since the 1940s. Nowadays, there are a variety of these injectable vaccines, all with similar potential side effects. The most frequently reported side effect is soreness at the injection site. When looking at the results of a number of studies, the CDC found that 10 to 64 percent of adults experience some soreness, which typically lasts less than 2 days and does not interfere with daily activities. Localized redness and swelling may accompany the pain. Two vaccines -- one with a stronger virus dose and the other given just under the skin instead of into muscle -- are especially likely to cause injection site effects.
Other Common Side Effects
Adults may occasionally faint after receiving any injection, including a flu shot. Headache, fever, fatigue, malaise, muscle aches and nausea are other transient side effects that some people experience after being immunized. Since injectable flu vaccines are all created using killed influenza virus, it is not possible to catch the flu from a flu shot. However, it takes on average about 2 weeks for the vaccination to become fully effective, so it is possible to catch the flu before the shot has had time to work.
Allergic reactions can occur to any component in the flu vaccine and may range from mild to severe. Anaphylaxis is a severe reaction that produces hives, wheezing, swelling of the mouth or face, difficulty breathing or loss of consciousness. Anyone with suspected anaphylaxis requires immediate medical attention. The CDC reports that anaphylaxis is rare, with less than 2 cases reported for every 1 million doses of flu vaccine given. As chicken eggs are used to make most flu vaccines, people with egg allergies may receive regular flu shots but should first discuss the situation with their doctor, since special precautions may be required. In 2016, the CDC published updated precautions for individuals with egg allergies. Recently, flu vaccines have been developed that do not use any eggs in the manufacturing process.
Possible Rare Effect
Guillain-Barré syndrome is a very rare condition in which a person's immune system attacks their nerves. This leads to progressive muscle weakness and sometimes paralysis. Guillain-Barré syndrome can occur after various types of viral or bacterial infections, including the flu. While research into the risk of developing the syndrome after a flu shot is ongoing, the CDC estimates that 1 case of Guillain-Barré syndrome may occur for every 1 million doses of flu vaccine given. People are more likely to develop Guillain-Barré if they become infected with the flu than if they receive a flu shot. However, people who have recently been ill with Guillain-Barré syndrome should discuss their situation with their doctor before being immunized and anyone who suspects they have the syndrome should seek prompt medical evaluation.
Reviewed by: Mary D. Daley, M.D.
- Centers for Disease Control and Prevention: Prevention and Control of Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices, United States, 2015–16 Influenza Season
- Centers for Disease Control and Prevention: Vaccine Effectiveness
- Adverse Effects of Vaccination: Evidence and Causality; Kathleen Stratton, et al.
- PLoS ONE: The Decision to Vaccinate or Not During the H1N1 Pandemic: Selecting the Lesser of Two Evils?
- World Health Organization: Vaccines Against Influenza WHO Position Paper – November 2012
- Centers for Disease Control and Prevention: Key Facts About Seasonal Flu Vaccine
- Centers for Disease Control and Prevention: Misconceptions About Seasonal Flu and Flu Vaccines
- Centers for Disease Control and Prevention: Safety of Influenza Vaccines
- Centers for Disease Control and Prevention: Flu Vaccine and People With Egg Allergies
- Centers for Disease Control and Prevention: Influenza (Flu) Vaccines Are Made
- Centers for Disease Control and Prevention: Guillain-Barré Syndrome and Flu Vaccine