Flu vaccines trigger the body’s immune system to make antibodies that attack the viruses that are in the vaccine. Antibodies are proteins that recognize and fight off germs. So, if a vaccinated person is later exposed to one of the viruses, the immune system will recall the virus and use antibodies to fight it.
Each year, the Food and Drug Administration (FDA) decides which vaccine viruses to include in the flu vaccines to be sold in the U.S. The decision is based on surveillance data that shows which viruses are circulating and predicts which viruses are the most likely to circulate during the next flu season.
“There are many flu viruses and they are constantly changing,” according to the Center for Disease Control and Prevention (CDC). “Flu vaccines protect against the three or four viruses that research suggests will be most common during the upcoming season.”
When a vaccine has three components, it is known as trivalent; when it has four components, it’s known as quadrivalent. People who get a flu shot can still get the flu that same season. Various factors, such as the age and the health status of the person being vaccinated, as well as whether the viruses used to make the vaccine match those circulating in the community, can affect how well the flu vaccine works.
Flu vaccines only protect against the three or four viruses that are most common during the upcoming season.
It takes about two weeks after vaccination for the body to make antibodies and provide protection against the flu. The CDC recommends people get vaccinated before flu season is in full swing. Flu season in the U.S. is considered October through May. Flu virus protection in the body decreases over time, so a flu vaccine is needed every season for best protection. Plus, flu vaccines are sometimes updated to reflect changes in flu viruses.
Each year, people should start getting the flu vaccine as soon as it becomes available — preferably by the end of October, according to the CDC. Private manufacturers produce the flu vaccines, and the timing of when the vaccines are made available is based on when production is completed. Flu vaccines should continue to be offered throughout the flu season, the CDC says.
Who Should/Should Not Get the Flu Shot?
Different flu shots are approved for different ages. Some are licensed for use in people as young as 6 months of age and older. Others, like the high-dose flu shot and the flu shot with adjuvant (a substance that is added to a vaccine to increase the body’s immune response to the vaccine), should not be used in people younger than 65 years of age. Moreover, people who are younger than 18 years old or older than 64 years old should not get the intradermal flu shot, which is injected into the skin instead of the muscle. Children younger than 6 months are too young to get any flu shot.
Pregnant women and people with chronic health conditions can get flu vaccines. People with severe, life-threatening allergies to the flu vaccine or any ingredient in the vaccine, including gelatin and antibiotics, should not get the flu vaccine.
People who should talk to their doctor before getting the flu shot include:
- Anyone with an allergy to eggs or any of the ingredients in the vaccine
- Anyone who has had Guillain-Barré syndrome (GBS), which is a severe paralyzing illness
- Anyone who is not feeling well at the time of the scheduled vaccination
Any of these might be reason to delay vaccination or to not get the flu vaccine at all. The patients’ doctors will advise them.
Flu Vaccine and Egg Allergies
Manufacturers use egg-based technology to make most flu shots and the nasal spray vaccine. As a result, flu vaccines contain a small amount of egg proteins. For some time, the CDC recommended anyone with a history of severe allergic reaction to egg (any symptom other than hives) be given a flu vaccine only by a doctor with experience in managing severe allergic conditions. The agency also recommended the patient be observed for a half hour following vaccination.
However, studies show it is unlikely people with egg allergies will have severe allergic reactions to flu vaccines. A recent CDC study, for example, indicated the rate of severe allergic reaction after all vaccines is 1.31 per one million vaccine doses given.
Rate of Reaction
The rate of severe allergic reaction after all vaccines is 1.31 per one million vaccine doses given.
The CDC and its Advisory Committee on Immunization Practices have updated their guidelines on egg allergy and getting a flu vaccine.
“Based on the new recommendations, people with egg allergies no longer need to be observed for an allergic reaction for 30 minutes after receiving a flu vaccine,” according to the CDC.
In addition, people with a severe allergic reaction to egg can now get a flu vaccine at hospitals, clinics, health departments and doctor’s offices “under the supervision of any health care provider who is able to recognize and manage severe allergic conditions,” the CDC says.
Flu Vaccine Side Effects and Risks
Any side effects people suffer after the flu shot are usually mild. Though, in rare cases, people experience severe, life-altering reactions. Before getting the flu shot, it’s important to know the risks. The U.S. Department of Health and Human Services manages the National Vaccine Injury Compensation Program (VICP), which is a program that identifies certain vaccines and the potential illnesses, injuries and disabilities they cause, and provides compensation to people who file injury and death petitions and are found to have been injured by the vaccines.
VICP covers most vaccines given routinely in the U.S., including flu vaccines. Injuries related to the flu vaccine that are covered by the compensation program include anaphylaxis, shoulder injury related to vaccine administration (SIRVA), vasovagal syncope and Guillain-Barré syndrome.
Anaphylaxis (Severe Allergic Reaction)
The VICP defines anaphylaxis as “an acute, severe and potentially lethal systemic reaction that occurs as a single discrete event with simultaneous involvement of two or more organ systems.” In other words, anaphylaxis is a severe, life-threatening allergic reaction that needs to be treated right away.
Warning signs and symptoms of anaphylaxis usually emerge minutes to a few hours after vaccination. For purposes of compensation, the reaction must start within four hours of receipt of the flu vaccine.
Signs and symptoms of anaphylaxis include:
- Hives and itching
- Skin reactions and discoloration
- Low blood pressure
- Swollen tongue or throat with wheezing and trouble breathing
- Abnormally slow or rapid heartbeat
Most cases don’t result in long-term, negative aftereffects; however, death is possible. If death occurs it’s usually the result of a blocked airway caused by swelling of the larynx (the voice box) or sudden constriction of the muscles in the walls from the nose or mouth to the air sacs of the lungs. Death can also result from sudden loss of blood flow to the brain and other organs, known as cardiovascular collapse.
Shoulder Injury Related to Vaccine Administration (SIRVA)
Some people experience pain in the arm and shoulder in which the vaccine was given. If pain or weakness occurs within 48 hours of vaccination, then the patient may be eligible for compensation through the VICP.
Brachial Neuritis Pains
If pain or weakness occurs within 48 hours of vaccination, then the patient may be eligible for compensation through the VICP.
A shoulder injury related to the flu vaccine known as brachial neuritis is a VICP-covered injury. This condition is characterized by inflammation of the brachial plexus, which is network of nerves that start near the neck and shoulder. It usually starts with a deep, steady and often severe pain in the shoulder and upper arm followed by weakness and possibly sensory loss. Brachial neuritis can also make it difficult to move the arm and/or shoulder and can cause the muscle to waste away. These symptoms may affect the side in which the flu shot was given, the side opposite the injection or both.
Vasovagal Syncope (Fainting)
Another potential reaction to the flu vaccine is vasovagal syncope, the most common type of fainting. Anyone who faints within one hour of getting the flu vaccine may be eligible for compensation through VICP.
Vasovagal syncope may lead to falling and injury with significant long-term aftereffects. People may become pale and experience nausea, lightheadedness and sweating before fainting. It may be also accompanied by temporary seizure-like activity.
For purposes of compensation, loss of consciousness resulting from the following conditions will NOT be considered vasovagal syncope:
- Organic heart disease
- Transient ischemic attacks
- Metabolic conditions
- Cardiac arrhythmias
- Neurological conditions
Guillain- Barré syndrome (GBS) is a potentially life-threatening, rare condition in which a person’s immune system attacks the peripheral nerves. People with GBS should be treated and monitored, and may need intensive care. GBS starts with weakness or tingling sensations in the legs and can spread to the arms and face. Symptoms can cause paralysis of the legs, arms, face muscles and chest muscles. It can make it hard to breathe, speak and swallow.
Most people recover fully, some continue to experience weakness and as many as 5 percent of GBS patients die from complications, which can include paralysis of the muscles that control breathing, blood infection, lung clots or cardiac arrest.
If GBS emerges between three and 42 days following flu vaccination, then the patient may be eligible for compensation through the VICP.
The following four subtypes of GBS are also covered by the VICP:
- Acute inflammatory demyelinating polyneuropathy (AIDP)
- Acute motor axonal neuropathy (AMAN)
- Acute motor and sensory neuropathy (AMSAN)
- Fisher syndrome (FS), also known as Miller Fisher Syndrome
Emily Miller is an award-winning journalist who has held writing and editorial positions with numerous print and online publications around the U.S. She draws on her background as both a patient and a journalist to help readers understand complex health topics. Her most recent experience is in writing about litigation involving medicines that cause life-altering side effects.
- U.S. Department of Health and Human Services Health Resources and Services Administration. (2017, March). National Vaccine Injury Compensation Program. Retrieved from https://www.hrsa.gov/vaccinecompensation/
- U.S. Department of Health and Human Services Health Resources and Services Administration. (n.d.). Vaccine Injury Table. Retrieved from https://www.hrsa.gov/vaccinecompensation/vaccineinjurytable.pdf
- CDC.gov. (2016, August 25). Seasonal Flu Shot. Retrieved from https://www.cdc.gov/flu/about/qa/flushot.htm
- CDC.gov. (2016, September 2). Flu Vaccine and People with Egg Allergies. Retrieved from https://www.cdc.gov/flu/protect/vaccine/egg-allergies.htm
- CDC.gov. (2017, April 11). Frequently Asked Flu Questions 2017-2018 Influenza Season. Retrieved from https://www.cdc.gov/flu/about/season/flu-season-2017-2018.htm
- World Health Organization. (2016, October). Guillain–Barré syndrome. Retrieved from http://www.who.int/mediacentre/factsheets/guillain-barre-syndrome/en/