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Questions & Answers

How Well Does the Seasonal Flu Vaccine Work?

Seasonal Influenza Vaccine Effectiveness

How effective is the flu vaccine?

How well the flu vaccine works depends on how well the match is between the influenza (flu) vaccine and the types of flu viruses that are circulating that year. Scientists try to predict what strains (types) of flu viruses are most likely to spread and cause illness each year to put into the vaccine. In years when the vaccine strains and the virus strains are well-matched, the vaccine can reduce the chances of getting the flu by 70%-90% in healthy adults. The vaccine may be somewhat less effective in elderly persons and very young children, but vaccination can still prevent serious complications from the flu.

In healthy adults less 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. However, the 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 flu vaccine won't protect you from cold or flu viruses that are already in your body when you get a flu vaccine. The flu vaccine takes about two weeks to provide protection from the flu, and it's your best protection to prevent the most common types of flu this season.

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 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.

Another reason to get the flu vaccine every year is that after vaccination, protection from the flu strains in the vaccine decreases over the year.

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

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 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. Among elderly nursing home residents, the flu shot is most effective in preventing severe illness, 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 the 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 less 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 less than 6 months of age are most at risk for having complications from 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.

How are the viruses for flu vaccine chosen?

Each year, experts study thousands of flu virus samples from around the world to figure out which viruses are making people sick and how these viruses are changing; with this information, they forecast which three viruses are most likely to make the most people sick during the next flu season. Other considerations include1) how well the current vaccine protects against the viruses forecasted to cause a lot of illness in the next season and2) if these viruses can be used to make a new vaccine if needed. As soon as experts decide which three viruses should be used to create the next season’s vaccine, vaccine producers begin making vaccine.

What if the viruses used to make the vaccine are different from those causing most illness?

CDC recommends getting a flu vaccination every year, even when there is a less than ideal match between the viruses used to make the vaccine and those causing illness. This is especially important for people at higher risk for serious flu-related complications and for people who come into close contact with them.

Flu viruses are always changing. Sometimes they change from when the vaccine is recommended and the beginning of the flu season; they can even change during a flu season. When flu viruses change, they may no longer closely match viruses used to make that season’s flu vaccine. This can make the vaccine less effective. But even when this happens, the vaccine can still offer protection:

  1. The vaccine contains three viruses, so it can protect you against the other two viruses that may be making people sick.
  2. The immune protection you get from the vaccine can provide partial protection against flu viruses that are related to those used to make the vaccine (this is called cross-protection.) So while a less than ideal match can reduce vaccine benefit, the vaccine can still provide enough protection to make illness less severe and prevent flu-related complications.

A less than perfect vaccine is still the best protection we have against influenza. That is why CDC continues to recommend getting the vaccine even when there is a less than perfect match between viruses used to make the vaccine and viruses making a lot of people sick.

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

Getting the flu vaccine each year is the best way to prevent the flu. Antiviral drugs also can be used to treat and prevent 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.

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