Influenza Vaccine (Flu Shot) Facts, Vaccination Side Effects and Complications by MedicineNet.com

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Flu Vaccination (cont.)

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 vaccines are routinely available for seasonal influenza, but these vaccines are not likely to be protective against flu caused by the novel H1N1 (formerly termed swine flu) virus. A vaccine against the novel H1N1 flu is in production as of summer 2009 and may be ready for administration in fall of 2009.

There are two types of seasonal flu vaccines, the injection (with killed virus) and nasal spray vaccines (containing live but weakened virus).

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 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.

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.

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 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.

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. The vaccine is given annually, each fall. Side effects of the inactivated flu vaccine are not common.

Side effects include soreness at the site of the injection, muscle aching, fever, and feeling unwell. Very rarely, serious allergic reactions have been reported.

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 been done to evaluate 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.

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 2-49 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

children younger than 5 years of age who have recurrent wheezing;

people with chronic health problems, including heart and lung disease;

pregnant women;
people with suppressed immune function and those who care for or come into contact with those with a suppressed immune system;

adults over age 50 or children 6 months to 2 years of age;

children or adolescents receiving aspirin therapy.

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 efficacy 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 recent 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.