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Medical Author:

Melissa Conrad Stöppler, MD

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.

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Medical Editor:

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.

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In this Article

• Flu vaccine (flu shot) facts
• What is flu?
• Why vaccinate for the flu?
• What are the different types of flu vaccines?
• What flu viruses does the flu vaccine protect against?
• How does the flu vaccine work to prevent the flu?
• When should one receive the flu vaccine?
• Who should receive the flu vaccine?
• Who should not receive the flu vaccine?
• What are risks and side effects of the flu vaccine?
• Can the flu vaccine give me the flu?
• What should I do about adverse reactions to the flu vaccine?
• How effective is the flu shot?
• What was the novel H1N1 (swine flu) vaccine?
• What is the best way to locate a flu shot clinic?
• Patient Comments: Flu Vaccination - Side Effects
• Flu Vaccination Index

Why vaccinate for the flu?

The flu is highly infectious and is a serious viral respiratory infection. 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. 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 the H1N1 virus. Tested and approved, H1N1 vaccine first became available in October 2009 in the Americas.

There are two types of seasonal flu vaccines, the injection (with killed virus) and nasal spray vaccines (containing live but weakened virus). A newer type of injection vaccine was introduced during the 2011-2012 flu season, a vaccine that can be injected into the skin (intradermally) rather than into the muscle (intramuscularly).

Each year, the influenza virus can change slightly, making the vaccine used in previous years ineffective. Each year, a new vaccine must be prepared that will be effective against the expected type of influenza virus. 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. The viruses that are used to prepare flu vaccine are grown 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. The killed influenza virus is injected 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, the injection is into the deltoid muscle at the side of the arm, using alcohol rubbed over the skin for sterilization. A newer preparation first introduced in the 2011-2012 flu season is an intradermal vaccine that is injected into the skin rather than into the muscle. The intradermal vaccine is approved for people 18-64 years of age. The vaccine is given annually, each fall. Side effects of the inactivated flu vaccine are not common.

Additionally, a special high-dose vaccine has been developed for people 65 years of age and older. The higher dose is designed to elicit a stronger immune response in this age group, since older people typically have weakened immune responses compared with younger people.

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 viruses are weakened so that they do not cause severe flu symptoms. The nasal spray flu vaccine (LAIV) has been approved by the U.S. Food and Drug Administration (FDA) for use in nonpregnant healthy people between 2 to 49 years of age.

People at risk for serious complications from the flu should not receive the nasal spray flu vaccine. In particular, certain groups are advised to receive the inactivated flu vaccine rather than the nasal spray vaccine, including

• people less than 2 years of age;
• people 50 years of age and over;
• people with a medical condition that places them at high risk for complications from influenza, including those with chronic heart or lung disease, such as asthma or reactive airways disease;
• people with medical conditions such as diabetes or kidney failure;
• people with illnesses that weaken the immune system or who take medications that can weaken the immune system;
• children or adolescents receiving aspirin;
• pregnant women;
• people who have a severe allergy to chicken eggs or who are allergic to any of the nasal-spray vaccine components;
• 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.

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 include runny nose, headache, sore throat, and cough. Children who receive the vaccine may also develop mild fever and muscle aches.

Several studies have shown excellent effectiveness with respect to prevention of flu in children with the nasal vaccine, even better than the injectable vaccine. Among adults, efficacy of the injectable vaccine varies from year to year, but one study demonstrated that injectable vaccine was substantially more efficacious than nasal vaccine at preventing flu. If this difference in efficacy can be confirmed, it would support the use of injectable vaccine over nasal vaccine among adults who have no medical conditions that would prevent them from receiving the injectable vaccine.