Flu vaccines are routinely available for seasonal influenza, but these vaccines are not likely to be protective against flu caused by the H1N1 (formerly termed swine flu) virus. There is a separate vaccine against the H1N1 flu.
There are two types of seasonal flu vaccines, the injection (with killed virus) and nasal spray vaccines (containing live but weakened virus).
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.
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). 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.
The nasal-spray vaccine
The nasal-spray flu vaccine (sometimes called LAIV for live attenuated influenza vaccine, brand name FluMist) 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 FDA for use in nonpregnant healthy people between two and 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 five 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 six months to two 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.
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