Vaccine Effectiveness - How Well Does the Flu Vaccine Work?
Questions & Answers
How well the flu vaccine works (or its ability to prevent flu illness) can range widely from season to season. The vaccine’s effectiveness also can vary depending on who is being vaccinated. At least two factors play an important role in determining the likelihood that flu vaccine will protect a person from flu illness: 1) characteristics of the person being vaccinated (such as their age and health), and 2) the similarity or "match" between the flu viruses the flu vaccine is designed to protect against and the flu viruses spreading in the community. During years when the flu vaccine is not well matched to circulating viruses, it’s possible that no benefit from flu vaccination may be observed. During years when there is a good match between the flu vaccine and circulating viruses, it’s possible to measure substantial benefits from vaccination in terms of preventing flu 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.
Each season researchers try to determine how well flu vaccines work to regularly assess and confirm the value of flu vaccination 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 flu 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 flu vaccination benefits public health, especially when the flu vaccine is well matched to circulating flu viruses.
Seasonal flu vaccines are designed to protect against infection and illness caused by the flu viruses research indicates will be most common during the flu season. “Trivalent” flu vaccines are formulated to protect against three flu viruses, and “quadrivalent” flu vaccines protect against four flu viruses. Flu vaccines do NOT protect against infection and illness caused by other viruses that can also cause flu-like symptoms. There are many other viruses besides flu viruses that can result in flu-like illness* (also known as influenza-like illness or “ILI”) that spread during the flu season.
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 flu vaccine is with circulating viruses). In general, the flu vaccine works best among healthy adults and older children. Some older people and people with certain chronic illnesses might develop less immunity than healthy children and adults after vaccination. However, even for these people, the flu vaccine still may provide some protection.
Older people with weaker immune systems often have a lower protective immune response after flu vaccination compared to younger, healthier people. This can result in lower vaccine effectiveness in these people.
In general, the flu vaccine works best among healthy adults and children older than 2 years of age. Reduced benefits of flu vaccine are often found in studies of young children (e.g., those younger than 2 years of age) and older adults (e.g., adults 65 years of age and older).
Public health researchers measure how well flu vaccines work through different kinds of studies. “Randomized studies,” in which people are randomly assigned to receive either vaccine or placebo (i.e., salt water solution), and then followed to see how many in each group get the flu, are the “gold standard” (best method) for determining how well a vaccine works. The effects of vaccination measured in these studies is called “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 effects in an observational study is referred to as “effectiveness.” Randomized studies are expensive and are not 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, most U.S. studies conducted to determine the benefits of flu vaccination in the elderly are observational studies.
CDC typically presents vaccine effectiveness (VE) as a single point estimate: for example, 60%. This point estimate represents the reduction in risk provided by the flu vaccine. CDC vaccine effectiveness studies commonly measure laboratory confirmed flu illness that results in a doctor’s visit or urgent care visit as an outcome. For this outcome, a VE point estimate of 60% means that the flu vaccine reduces a person’s risk of developing flu illness that results in a visit to the doctor’s office or urgent care provider by 60%.
In addition to the VE point estimate, CDC also provides a “confidence interval” (CI) for this point estimate, for example, 60% (95% CI: 50%-70%). The confidence interval provides a lower boundary for the VE estimate (e.g., 50%) as well as an upper boundary (e.g., 70%). One way to interpret a 95% confidence interval is that if CDC were to repeat this study 100 times, 95 times out of 100, the VE point estimate would fall within the confidence interval (i.e., on or between 50% and 70%). There is still the possibility that five times out of 100 (a 5% chance) that CDC’s point estimate of VE could fall outside of the 50%-70% confidence interval.
Confidence intervals are important because they provide context for understanding the precision or exactness of a VE point estimate. The wider the confidence interval, the less exact the point value estimate of vaccine effectiveness becomes. Take, for example, a VE point estimate of 60%. If the confidence interval of this point estimate is 50%-70%, then we can have greater certainty that the true protective effect of the flu vaccine is near 60% than if the confidence interval was 10-90%. Furthermore, if a confidence interval crosses zero, for example, (-20% to 60%), then the point value estimate of VE provided is “not statistically significant.” People should be cautious when interpreting VE estimates that are not statistically significant because such results cannot rule out the possibility of zero VE (i.e., no protective benefit). The width of a confidence interval is related in part to the number of participants in the study, and so studies that provide more precise estimates of VE (and consequently, have a tighter confidence interval) typically include a large number of participants.
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 considered.
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 flu season to estimate how well flu vaccine works through observational studies using laboratory-confirmed flu as the outcome. These studies currently use a very accurate and sensitive laboratory test known as RT-PCR (reverse transcription polymerase chain reaction) to confirm medically-attended flu 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 flu vaccination. Similar studies are being done in Australia, Canada and Europe.
CDC conducts studies each year to determine how well the flu vaccine protects against flu 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 circulating flu viruses are like the viruses the flu vaccine is designed to protect against.
The large numbers of flu-associated illnesses and deaths in the United States, combined with the evidence from many studies showing that flu vaccines help to provide protection, support the current U.S. flu 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 the flu vaccine and the flu viruses that are spreading and causing illness in the community, as well as the characteristics of the person being vaccinated.
CDC has compiled a list of selected publications related to vaccine effectiveness.
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 frequently washing your hands with soap, can help prevent the spread of the flu and other respiratory illnesses.
More information on Vaccine Selection.