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Vaccine Effectiveness

How Well Does the Flu Vaccine Work?

How effective is the flu vaccine?

Inactivated influenza vaccine effectiveness (VE) can vary from year to year and among different age and risk groups. In general, the highest VE is among young healthy adults and older children. Lower VE is reported for young children (e.g., those younger than 2 years of age), and older adults with only one randomized trial conducted in adults 60 years and older. VE also depends on the degree to which influenza vaccine viruses are well matched to circulating influenza viruses. Lower VE is expected in years when the match between circulating and vaccine viruses are suboptimal. VE also varies based on the outcome used, with generally the lowest VE estimates using non-influenza specific non-laboratory confirmed outcomes. The use of antibody seroconversion as a sole measure of VE may result in an overestimation of VE compared to other laboratory outcomes including viral culture and RT-PCR. CDC is currently reviewing recently published studies on VE to update existing estimates.

In healthy adults younger than 65 years of age, the flu vaccine can also prevent lost work days, and keep you from having to see the doctor or using unnecessary antibiotics.

Is the flu vaccine effective against all types of flu and cold viruses?

The flu vaccine is your best protection against flu viruses. The 2010-2011 flu vaccine will protect against the H1N1 virus that caused so much illness last flu season. The flu vaccine does not provide protection against non-flu viruses that can cause colds and other respiratory illnesses. It can sometimes be hard to tell the difference between a cold and the flu based on symptoms alone.

The 2010-2011 flu vaccine will protect against 2009 H1N1, and two other influenza viruses (an H3N2 virus and an influenza B virus).

Why do I need to get a flu vaccine every year?

Flu viruses change from year to year, which means two things. First, you can get the flu more than once during your lifetime. The immunity (natural protection that develops against a disease after a person has had that disease) that is built up from having the flu caused by one flu virus strain doesn't always provide protection against newer strains of the flu. Second, a flu vaccine made against flu viruses going around last year may not protect against the newer viruses. That is why the flu vaccine is updated to include current viruses every year.

Because of these reasons, a new flu vaccine is needed each year.

Should I get the 2010-11 flu vaccine even if I received the 2009 H1N1 vaccine late in the 2009-2010 flu season?

Yes. You need to get the 2010-11 seasonal flu vaccine even if you got the 2009 H1N1 flu vaccine late last season. The flu vaccine is updated each season to provide protection against the three flu viruses that research indicates will cause the most illness. The 2010-11 flu vaccine provides protection against the 2009 H1N1 virus, and 2 additional viruses (an influenza A H3N2 virus and an influenza B virus) that were not included in the 2009 H1N1 flu vaccine. So if you don't get the 2010-11 flu vaccine, you will not be protected against two of the three main flu viruses expected to circulate this flu season. Additionally, immunity wanes over time, so even if you received the 2009 H1N1 vaccine late in the 2009-10 flu season, you may not be protected against the 2009 H1N1 virus for the 2010-11 flu season.

Does getting a flu vaccine early in the season mean that I will not be protected later in the season?

Flu vaccination provides protection against the influenza strains contained in the vaccine that will last for the whole season. Vaccination can begin as soon as vaccine is available. Studies do not show a benefit of receiving more than one dose of vaccine during a flu season, even among elderly persons with weakened immune systems.

Does the flu vaccine work the same for everyone?

The flu vaccine is the single best way to prevent the flu, and vaccination is the main tool used to protect people from influenza. A number of studies have shown that the flu vaccine works, but how well the vaccine works can change from year to year and vary among different groups of people. The ability of the flu vaccine to protect a person depends on at least two things: 1) the age and health of the person getting the vaccine and, 2) the similarity or "match" between the virus strains in the vaccine and those being spread in the community.

Vaccine effectiveness is not 100%, and some people can still get the flu. For instance, some older people and people with certain chronic illnesses might develop less immunity than healthy young adults after vaccination. However, even for these high-risk individuals, the flu vaccine still can provide protection against getting severe complications from the flu.

How effective is the flu vaccine in the elderly?

Among elderly persons not living in chronic-care facilities (such as nursing homes) and those persons with long-term (chronic) medical conditions (such as asthma, diabetes, or heart disease), the flu shot is 30%-70% effective in preventing hospitalization for pneumonia (a lung infection) and influenza. In past studies among elderly nursing home residents, the flu shot was most effective in preventing severe illness and complications that may follow flu (like pneumonia), and deaths related to the flu. In this population, the shot can be 50%-60% effective in preventing hospitalization or pneumonia, and 80% effective in preventing death from the flu.

Because persons aged 65 years and older are at high risk for serious complications from flu, it also is important that people who live with or care for those at high risk for serious complications get a flu vaccination.

How effective is the flu vaccine in children?

Because children younger than 5 years of age are at increased risk of severe flu illnesses, children 6-59 months and the household contacts and caregivers of children 0-59 months are recommended to get the flu vaccine every year. Children younger than 6 months of age are most at risk for having complications from the flu. However, they are too young to get the flu vaccine. To protect these infants, it is very important that their household members and out-of-home caregivers be vaccinated against the flu.

The flu vaccine can prevent 66% or more influenza infections in young children, with even higher estimates for older children, when the vaccine strains are well-matched to the flu viruses causing illness. Vaccinating close contacts of children can also help decrease children’s risk of getting the flu.

All children 6 months through 8 years of age are recommended to receive 2 doses of 2010-11 flu vaccine 4 or more weeks apart unless they have received:

  1. at least 1 dose of 2009 H1N1 vaccine last flu season, and
  2. at least 1 dose of seasonal vaccine prior to the 2009-2010 flu season or 2 doses of 2009-10 seasonal flu vaccine last flu season
If a child has fulfilled both requirements, they only need 1 dose of the 2010-11 flu vaccine.

Studies have shown that two doses are needed in children younger than 9 the first year they are vaccinated in order to maximize the protective benefit from vaccination. Once you get vaccinated, your body makes protective antibodies in about two weeks. However, children younger than 9 years old who are being vaccinated for the first time need a second dose 4 or more weeks later in order to be protected.

Besides vaccination, how can people protect themselves against seasonal flu?

Getting the flu vaccine each year is the best way to prevent the flu. Antiviral drugs are an important second line of defense against the flu; these drugs must be prescribed by a doctor. In addition, good health habits, such as covering your cough and washing your hands, can help prevent the spread of flu and other respiratory illnesses.

More information on Selecting the Viruses in the Seasonal Influenza (Flu) Vaccine.

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