By Dr Prashant Pruthi, MD  (Paediatrics)
Every year, on an average, 5% to 20% of the population gets the flu; many persons are hospitalized from flu complications; and people may die from flu. The flu is highly infectious and is a serious viral respiratory infection. Flu vaccine is an inactivated vaccine, meaning that it contains killed influenza virus. Anyone who wishes to reduce their risk of getting the flu can be vaccinated; however, the US Centers for Disease Control and Prevention recommends that at least people who are at risk for serious complications from the flu should be vaccinated every year.
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 a higher grade fever, more malaise, and more severe body aches than other respiratory infections. Although other viruses may cause these symptoms, they do so less commonly.
Why vaccinate for the flu?

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, super-infections may occur. Super-infections are bacterial infections that occur on top of a viral or other 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 causing 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 new H1N1 (formerly termed swine flu) virus.

The flu vaccine is generally effective against the influenza virus within two weeks of administration. The flu 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 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 the 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 under the skin (subcutaneous) or into muscle (intramuscular or IM). The dose of vaccine is 0.25ml for kids between six months and three years. The vaccine should be given every year. Side effects from the flu vaccine are not common.
Who should receive the flu vaccine?

While anyone who wishes to reduce their risk of getting the flu can be vaccinated, the US Centers for Disease Control and Prevention (CDC) recommends that certain groups of people (who are at risk for serious complications from the flu) should be vaccinated each year. The CDC recommends vaccination for the following groups:
Children aged 6 months up to 19 years of age
Pregnant women

People 50 years of age and older
People of any age with certain chronic medical conditions
People who live in nursing homes and other long-term care facilities
People who live with or care for those at high risk for complications from flu, including, health-care workers, household contacts of people at high risk for complications from the flu, household contacts and out-of-home caregivers of children less than 6 months of age (these children are too young to be vaccinated).
Who should not receive the flu vaccine?

Those who should avoid the flu vaccine include:
People with a history of allergic reactions to eggs
Those with a history of hypersensitivity to the vaccine
Infants less than 6 months of age
Those with recent febrile (having fever) illness (although you can be vaccinated if you have a cold or other mild illness without fever)
How effective is the flu vaccine?

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. Research has shown that when there is a good match between the virus strains chosen for the vaccine and those in circulation, the vaccine prevents influenza illness in approximately 70%-90% of healthy adults under 65 years of age.
To conclude:
Each year, the influenza virus can change slightly, making the 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 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.