Flu Vaccine (Flu Shot)

Facts you should know about the influenza vaccine (flu shot)

The flu vaccine is effective against influenza 2 weeks after administration.
The flu vaccine is effective against influenza 2 weeks after administration.
  • Types of influenza (flu) vaccines include the injection (killed virus), recombinant (made without flu virus), and nasal spray vaccines (containing live virus).
  • Each year, influenza viruses change slightly, making the seasonal 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 seasonal flu 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. These other infections are usually categorized as URIs (upper respiratory infections) and have multiple causes. Influenza usually causes higher fever, more malaise, and severe body aches than other respiratory infections. Although other viruses may cause these flu-like 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. The CDC reports for the 2018/2019 influenza season show that in the United States

  • 5%-20% of the population gets the flu,
  • nearly 500,000 people are hospitalized from flu-related complications,
  • about 34,000 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.


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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 influenza vaccination. Although there are medications to treat the flu, they are expensive, not as effective as influenza 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 cause widespread disease, such as occurred with the H1N1 virus in 2009.

Each year, composition of the influenza viruses changes, 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 influenza 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. Flu viruses may also be made using recombinant technology that does not involve growing the actual flu virus in 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, or a recombinant vaccine, meaning that it was made without using virus particles at all. Health care providers inject the vaccine into muscles or skin, stimulating the immune system to produce an immune response (antibodies) to the influenza virus.

Medical professionals administer the "flu shot" vaccine as a single dose 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 flu vaccine are uncommon.

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. Special vaccines preparations are available for people over 65 years of age that produce a stronger immune response.

The U.S. Centers for Disease Control and Prevention (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 the influenza season of 2019-20.

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 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 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 includedrunny 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 immune system to produce 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.

Will the flu vaccine help fight the coronavirus?

The coronavirus (SARS-CoV-2) and influenza are two different illnesses caused by different viruses. The flu shot will not help fight coronavirus infection, but getting a flu shot remains an important step in protecting your health and in reducing the overall burden of respiratory illness in the community. Having enough people vaccinated against the flu ensures that hospitals and clinics keep adequate space to treat patients with coronavirus infections. It is expected that both the coronavirus and the flu may be present at the same time in the 2020-21 flu season, and being vaccinated against the flu helps protect yourself and vulnerable populations against an illness that might happen at the same time as a coronavirus infection.

Can you get COVID-19 and the flu at the same time?

According to the CDC, "While it's not possible to say with certainty what will happen in the fall and winter (of 2020-21), CDC believes it's likely that flu viruses and the virus that causes COVID-19 will both be spreading. In this context, getting a flu vaccine will be more important than ever." There is no evidence to suggest that it is not possible to have both diseases at the same time.

Is it safe to have a coronavirus vaccine if you already had a flu vaccine?

There is no coronavirus vaccine currently available, but vaccine developers are working to ensure development of a vaccine that is safe for everyone, including those who have had flu shots. Currently in the absence of a coronavirus vaccine, it is especially important for the 2020-21 flu season to receive a flu shot for its overall health benefits.

When should one receive the flu vaccine?

Health care professionals recommend getting the influenza vaccination 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.

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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.

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 vaccine (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 vaccine 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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United States. Centers for Disease Control and Prevention. "Frequently Asked Influenza (Flu) Questions: 2020-2021 Season." Aug. 31, 2020. <https://www.cdc.gov/flu/season/faq-flu-season-2020-2021.htm>.