Flu Vaccine (Influenza Immunization or Flu Shot)

  • Medical Author:
    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

Influenza vaccine (flu shot) facts

  • Types of influenza (flu) vaccines include the injection (killed virus) and nasal spray vaccines (containing live virus in recent years, but has been reinstated for certain people in 2018-2019).
  • Each year, influenza viruses change slightly, making the vaccine used in previous years ineffective.
  • The vaccine is generally effective against the influenza virus within two weeks of administration.
  • The vaccine is only effective against the strains of the virus that match the vaccine.
  • The vaccine effectiveness is dependent upon the extent of the match between the virus strains used to prepare the vaccine and those viruses in actual circulation in the community. The age and health status of the individual also play a role in determining the vaccine effectiveness.

What is influenza (flu)?

The flu (or common flu) is a viral infection that spreads from person to person in secretions of the nose and lungs, for example when sneezing. Medically, the common flu is another name for influenza. Flu is a respiratory infection, that is, an infection that develops primarily in the lungs. People often call respiratory infections caused by other viruses the flu, but this is incorrect. Influenza usually causes higher fever, more malaise, and severe body aches than other respiratory infections. Although other viruses may cause these symptoms, they do so less frequently.

Influenza viruses are divided scientifically into three types, designated A, B, and C. Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter. Influenza type C usually causes either a very mild respiratory illness or no symptoms at all; it does not cause epidemics and does not have the severe public health impact of influenza types A and B. Type A viruses are divided into subtypes and are named based on differences in two viral surface proteins called hemagglutinin (H) and neuraminidase (N). There are 16 known H subtypes and nine known N subtypes.

The flu is a common illness. Every year in the United States, on average

  • 5%-20% of the population gets the flu,
  • more than 200,000 people are hospitalized from flu complications,
  • about 36,000 people die from the flu or its complications.

The so-called swine flu pandemic of 2009 was caused by a novel influenza A virus designated H1N1 based upon its surface protein types. This virus was originally referred to as swine flu because many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs in North America. However, this virus was actually quite different from the typical swine flu viruses found in pigs.

Flu Shot Side Effects

Many people worry about side effects from the flu shot, but serious complications are rare. Some people believe that they can actually get the flu from receiving the shot, but this is not the case. For the majority of people, the risks of developing the flu are far greater than any risks associated with the vaccine.

Why vaccinate for the flu?

The flu is highly infectious and is a potentially serious viral respiratory infection that can even be life threatening. Whereas with other viral respiratory infections the symptoms usually are mild and most people can continue working or going to school while ill, with the flu, the symptoms are severe and prolonged and cause individuals to miss days of work or school. The infection stresses the body. In addition, superinfections may occur as a complication of the flu. Superinfections are bacterial infections that occur on top of a respiratory infection. Bacterial respiratory infections also are a serious type of infection, and the simultaneous viral and bacterial infection can overwhelm the function of the lungs and the body. Among the elderly and the very young, it can cause death. Because of its infectiousness, morbidity (severity of symptoms and time lost from work or school), and the potential for death, it is important to prevent the flu by vaccination. Although there are medications to treat the flu, they are expensive, not as effective as vaccination, and need to be started within 24-48 hours of the start of symptoms.

What are the different types of flu vaccines?

Flu vaccines are routinely available for seasonal influenza. Pandemic vaccines may also be developed for specific strains of the flu virus that are causing widespread disease, such as occurred with the H1N1 virus in 2009.

Two types of seasonal flu vaccines have been used, those using killed virus and nasal spray vaccines that contain live but weakened virus.

Each year, composition of the influenza viruses change, making the vaccine used in previous years ineffective. Each year, a new vaccine must be prepared that will be effective against the types of influenza virus that are expected to circulate in the upcoming flu season. These are known as seasonal flu vaccines. The reason for the differences in circulating strains of the flu virus is that the virus can mutate (or change its structure) rapidly, leading to new subtypes of the virus. The key is to be able to predict which influenza viruses are going to cause infection and to prepare a vaccine against those viruses. Usually, scientists can predict accurately which types of influenza virus will cause infections and prepare an appropriate vaccine. Typically, the viruses used to prepare flu vaccine are grown in eggs, but a newer, egg-free version of the vaccine has been developed. In 2017, updated guidelines from the Influenza Vaccine and Egg Allergy Practice Parameter Workgroup commissioned by the Joint Task Force on Practice Parameters (JTFPP) stated that the risk is so small that even asking patients about egg allergy is no longer necessary. Health care professionals can now safely administer flu vaccines to people who are allergic to eggs.

The vaccine is generally effective against the influenza virus within two weeks of administration. The vaccine is only effective against the strains of the virus that match the vaccine. These strains vary from flu season to flu season each year. This is the reason that revaccination is required annually with the vaccine that matches the strains of influenza that are currently prevalent.

The injection ("flu shot") vaccine

Flu vaccine is an inactivated vaccine, meaning that it contains killed influenza virus. Health care providers inject the killed influenza virus into muscles or skin and stimulates the immune system to produce an immune response (antibodies) to the influenza virus.

The inactivated flu vaccine is administered as a single dose of 0.5 mL of liquid injected through the skin into muscle (intramuscular or IM). Typically, health care professionals inject the flu vaccine into the deltoid muscle at the side of the arm, using alcohol rubbed over the skin for sterilization. Health care professionals administer the vaccine annually, each fall. Side effects of the inactivated flu vaccine are not common.

Two types of vaccines are available: a trivalent vaccine that targets three strains of flu virus, as well as a quadrivalent vaccine that targets four strains. Both the trivalent and quadrivalent vaccines are available as an intramuscular injection. For people who are 18-64 years old, a jet injector (that uses a strong stream of fluid to penetrate the skin rather than a needle) can be used for administration of certain vaccine types.

The U.S. CDC recommends the use of injectable influenza vaccines (either trivalent or quadrivalent, including inactivated influenza vaccines and recombinant influenza vaccines) or the nasal spray vaccine for 2018-19.

The nasal-spray vaccine

The nasal-spray flu vaccine (sometimes called LAIV for live attenuated influenza vaccine, brand name FluMist) was first licensed in 2003. It is directed against the same strains of virus as the flu shot but differs in that it contains weakened live influenza viruses instead of killed viruses and is administered by nasal spray instead of injection. The vaccine is termed an attenuated vaccine because the vaccine viruses are weakened so that they themselves do not cause severe flu symptoms. The nasal spray flu vaccine for 2018-19 is approved for use in non-pregnant individuals 2-49 years old. It should not be used for people with certain underlying medical conditions. All LAIV are quadrivalent (four-component).

The American Academy of Pediatrics (AAP) recommends that all children 6 months and older receive the injectable (shot) vaccine if possible. The AAP recommends the injectable flu vaccine for children because it has provided the most consistent protection against all strains of the flu virus in recent years. The nasal spray vaccine may be used this year for children who would not otherwise receive the flu shot (such as with a shortage of the injectable vaccine or refusal to receive an injection), as long as they are 2 years of age or older and healthy without an underlying medical condition.

The live viruses in the nasal-spray vaccine are weakened so that they do not cause severe symptoms. However, mild symptoms can occur as a side effect of the vaccination. Side effects of the nasal-spray flu vaccine can included runny nose, headache, sore throat, and cough. Children who receive the vaccine may also develop mild fever and muscle aches.

What flu viruses does the flu vaccine protect against?

Flu vaccines are developed each year and are designed to protect against the influenza viruses that are predicted to be the most common during the upcoming season. Some vaccines contain three viral strains (trivalent), while others contain four virus types (quadrivalent).

How does the flu vaccine work to prevent the flu?

The flu vaccines stimulate the production of antibodies in the body that fight the particular flu virus in the vaccination. When the virus enters a vaccinated person, the antibodies attack and kill the virus and prevent infection. Antibodies are produced against the specific strains of the virus contained in the yearly vaccine.

Flu vaccination does not protect against infection caused by microbes other than the influenza virus.

When should one receive the flu vaccine?

Health care professionals recommend getting the flu vaccine before flu season begins in your community. It takes about two weeks for the vaccine to produce a sufficient antibody response against the flu. Flu season can begin in October and last as late as May.

Who should receive the flu vaccine?

The CDC recommends that every individual over 6 months of age receive the seasonal flu vaccine. While everyone should get a vaccination, it is particularly important for some groups. Vaccination is especially important for people who are at high risk of developing serious complications if they get the flu, such as those with asthma, diabetes, and chronic lung disease as well as pregnant women and those over 65 years of age. It is also important for caregivers to get vaccinations, in addition to those who live with people in these risk groups.

Quick Guide10 Foods to Eat When You Have the Flu in Pictures

10 Foods to Eat When You Have the Flu in Pictures

Who should not receive the flu vaccine?

Those who should avoid the flu vaccine include the following:

  • People who have ever had a severe allergic reaction to influenza vaccine
  • People with a history of Guillain-Barré syndrome (a severe paralytic illness, also called GBS) that occurred after receiving influenza vaccine and who are not at risk for severe illness from influenza should generally not receive vaccine.
  • People under 65 years of age should not receive the high-dose flu shot.
  • If you are sick with a fever when you go to get your flu shot, you should talk to your doctor or nurse about whether or not you should get your shot later. However, you can get a flu shot at the same time you have a respiratory illness without fever or if you have another mild illness.
  • People who have had an allergic reaction to eggs can get any licensed flu vaccine that is otherwise appropriate for their age and health, but the vaccine should be given in a medical setting (including a doctor's office) and be supervised by a health care provider who is able to recognize and manage severe allergic conditions.

What are flu vaccine risks and side effects? Can the flu vaccine give me the flu?

Serious side effects of the flu vaccine are uncommon. Side effects of the injection vaccine include soreness at the site of the injection, muscle aching, fever, and feeling unwell. Very rarely, people have reported serious allergic reactions. The viruses in the nasal spray vaccine are weakened and do not cause severe symptoms; they cannot cause you to get the flu. Side effects from the nasal spray may include runny nose, mild fever, sore throat, cough, muscle aches, headache, and vomiting.

Guillain-Barré syndrome (GBS) is an illness characterized by fever, nerve damage, and muscle weakness. In 1976, vaccination with the swine flu vaccine was associated with development of GBS. Studies have evaluated if other flu vaccines were associated with GBS, with only one of the studies showing an association. That single study suggested that one person out of 1 million vaccinated people may be at risk of GBS associated with the vaccine.

What should I do about adverse reactions to the flu vaccine?

You should contact your health care professional in the case of any serious side effects. Mild side effects such as soreness at the injection site typically resolve on their own without treatment.

How effective is the flu shot?

The effectiveness of the flu vaccine is dependent upon the extent of the match between the virus strains used to prepare the vaccine and those viruses in actual circulation. The age and health status of the individual also play a role in determining the effectiveness of the vaccine. While vaccine effectiveness can vary, recent studies show vaccine reduces the risk of flu illness by about 40%-60% among the overall population during seasons when most circulating flu viruses are similar to those used in the vaccines. Similar reductions in the number of people hospitalized with the flu have also been observed. Rates of flu prevention may be even higher in healthy adults under 65 years of age.

What was the novel H1N1 (swine flu) vaccine?

The H1N1 vaccine was a pandemic vaccine designed to provide immunity against the novel H1N1 flu outbreak in 2009, initially referred to as swine flu. Initially, health care providers gave the H1N1 vaccine to those at highest risk for complications of this illness (children, pregnant women, caregivers of children). Even though the virus proved not to be as deadly as expected, researchers suggest the H1N1 vaccine was effective in reducing the effects of this flu virus.

What is the best way to locate a flu shot clinic?

People can obtain flu shots through a health care professional's office, at community health departments, and at many pharmacies. Additionally, many employers and schools host flu shot clinics. Some employers may offer the vaccine free of charge. A health care professional's office should be able to provide information about flu shot clinics available in the community.

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Medically Reviewed on 9/18/2018

American Academy of Pediatrics. "AAP flu vaccination recommendations 2018-19." Sept. 3, 2018. <https://www.aap.org/en-us/about-the-aap/aap-press-room/Pages/AAP-Issues-Flu-Vaccine-Recommendations-for-2018-2019.aspx>.

United States. Centers for Disease Control and Prevention. "Key Facts About Seasonal Flu Vaccine." Aug. 30, 2018. <https://www.cdc.gov/flu/about/season/flu-season-2018-2019.htm>.