Vaccine Effectiveness - How Well Does the Flu Vaccine Work?

Questions & Answers

How effective is the flu vaccine?

How well the flu vaccine works (or its ability to prevent influenza illness) can range widely from season to season and also can vary depending on who is being vaccinated. At least two factors play an important role in determining the likelihood that influenza vaccine will protect a person from influenza illness: 1) characteristics of the person being vaccinated (such as their age and health), and 2) the similarity or "match" between the influenza viruses in the vaccine and those spreading in the community. During years when the viruses in the vaccine and circulating viruses are not well matched, it’s possible that no benefit from vaccination may be observed. During years when the viruses in the vaccine and circulating viruses are very well matched, it’s possible to measure substantial benefits from vaccination in terms of preventing influenza illness. However, even during years when the vaccine match is very good, the benefits of vaccination will vary across the population, depending on characteristics of the person being vaccinated and even, potentially, which vaccine was used.

Researchers try to determine how well a vaccine works in order to continually assess and confirm the value of influenza vaccines as a public health intervention. Study results about how well a flu vaccine works can vary based on study design, outcome(s) measured, population studied and the season in which the vaccine was studied. These differences can make it difficult to compare one study’s results with another’s.

While determining how well a flu vaccine works is challenging, in general, recent studies have supported the conclusion that influenza vaccination benefits public health, especially when the viruses in the vaccine and circulating viruses are well-matched.

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

Influenza vaccines protect against infection and illness caused by the three influenza viruses in the vaccine that research indicates will be most common this season. Flu vaccines will NOT protect against infection and illness caused by other viruses that can also cause influenza-like symptoms. There are many other viruses besides influenza that can result in influenza-like illness* (ILI) that spread during the flu season.

Does the flu vaccine work the same for everyone?

No. While the flu vaccine is the single best way to prevent the flu, protection can vary widely, depending on who is being vaccinated (in addition to how well matched the vaccine viruses are with circulating viruses). In general, the flu vaccine works best among young healthy adults and older children. Some older people and people with certain chronic illnesses might develop less immunity than healthy young adults after vaccination. However, even for these people, the flu vaccine still may provide some protection.

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How effective is the flu vaccine in the elderly?

Older people with weaker immune systems often have a lower protective immune response after influenza vaccination compared to younger, healthier persons. This can result in lower levels of vaccine effectiveness in these people.

How effective is the flu vaccine in children?

In general, the flu vaccine works best among young, healthy adults and older children. Lesser effects of flu vaccine are often found in studies of young children (e.g., those younger than 2 years of age) and older adults.

How is vaccine effectiveness measured?

How well a vaccine works can be measured through different kinds of studies. “Randomized studies,” in which people are randomly assigned to receive either vaccine or placebo (e.g., saline solution), and then followed to see how many in each group get influenza, are the “gold standard” (best method) for determining how well a vaccine works. The measurement of vaccine effect from a randomized (placebo-controlled) study is referred to as “efficacy.” “Observational studies” are studies in which subjects who choose to be vaccinated are compared to those who chose not to be vaccinated. This means that vaccination of study subjects is not randomized. The measurement of vaccine effect from an observational study is referred to as “effectiveness.” Randomized studies are expensive and cannot be conducted after a recommendation for vaccination has been issued, as  withholding vaccine from people recommended for vaccination would place them at risk for infection, illness and possibly serious complications. For that reason, for example, most recent studies of how well flu vaccines work in the elderly have been observational studies.

Why are there so many different outcomes for vaccine effectiveness studies?

Results of studies that assess how well a flu vaccine works can vary based on study design, outcome(s) measured, population studied and the season in which the vaccine was studied. These differences can make it difficult to compare one study’s results with another’s. As there is interest in how well flu vaccines may prevent illness, hospitalization, and even death with influenza, many outcomes need to be used.

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How does CDC measure how well the vaccine works?

Scientists continue to work on better ways to design, conduct and evaluate non-randomized (i.e., observational) studies to assess how well flu vaccines work.CDC has been working with researchers at universities and hospitals since the 2003-2004 influenza season to estimate how well influenza vaccine works through observational studies using laboratory-confirmed influenza as the outcome. These studies currently use RT-PCR confirmed medically-attended influenza virus infections as a specific outcome. CDC’s studies are conducted in five sites across the United States to gather more representative data. To assess how well the vaccine works across different age groups, CDC’s studies of vaccine effects have included all people aged 6 months and older recommended at that time for an annual influenza vaccination. Similar studies are being done in Australia, Canada and Europe.

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What do recent vaccine effectiveness studies show?

CDC conducts studies each year to determine how well the vaccine protects against illness. These estimates provide more information about how well this season’s vaccine is working. Recent studies show vaccine can reduce the risk of flu illness by about 60% among the overall population during seasons when most characterized circulating influenza viruses are like the viruses included in the vaccine. Preliminary results for 2012-2013 are consistent with this. On January 11, 2013, CDC published interim early estimates of the 2012-2013 influenza vaccine’s effectiveness at preventing medical visits due to laboratory-confirmed influenza among people enrolled at 5 study sites across the United States. These estimates were adjusted for site, but were not adjusted for age or other potential confounders. CDC reported an overall VE of 62% with a 95% confidence interval of 51% to 71%. (Note: the confidence interval reflects how precise the VE estimate is.)

Do recent vaccine effectiveness study results support influenza vaccination?

The large numbers of influenza-associated illness and death in the United States, combined with the evidence from many studies showing that influenza vaccines help to provide protection, support the current U.S. influenza vaccination recommendations. It’s important to note, however, that how well flu vaccines work will continue to vary each year, depending especially on the match between influenza viruses used to make vaccine and the influenza viruses that are spreading and causing illness in the community as well as the characteristics of the person being vaccinated.

Where can I get more information?

CDC has compiled a list of selected publications related to vaccine effectiveness.

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

Getting a 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 the flu and other respiratory illnesses.

More information on Vaccine Selection.

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