Flu Vaccine
(Influenza Immunization)

Medical Revising Author: Melissa Conrad Stöppler, MD
Medical Revising Editor: Jay W. Marks, MD

• What is flu?
• Why vaccinate for the flu?
• What is the flu vaccine?
• Who should receive the flu vaccine?
• How is the flu vaccine administered?
• What side effects can occur with flu vaccination?
• Who should not receive the flu vaccine?
• How effective is the flu vaccine?
• What is the nasal spray flu vaccine?

What is flu?

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

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

• 5% to 20% of the population gets the flu,
  
  
• more than 200,000 people are hospitalized from flu complications, and



• about 36,000 people die from the flu.

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 is the flu vaccine?

Flu vaccine is an inactivated vaccine, meaning that it contains killed influenza virus. The killed influenza virus is injected into muscles and stimulates the immune system to produce an immune response (antibodies) to the influenza virus. When the virus enters a person who has been vaccinated, the antibodies attack and kill the virus and prevent infection.

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. The trick 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 the injection. 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.

Flu season can begin in October and last as late as May. October and November are considered the best times to receive the vaccination, but it is still effective when administered later.

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

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