Vaccine Effectiveness - How Well Does the Flu Vaccine Work?
Questions & Answers
CDC conducts studies each year to determine how well the flu vaccine protects against flu illness. While vaccine effectiveness can vary, recent studies show vaccine reduces the risk of flu illness by about 50% to 60% among the overall population during seasons when most circulating flu viruses are like the vaccine viruses.
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.
While how well the flu vaccine works can vary, there are a lot of reasons to get a flu vaccine each year.
- Flu vaccination can keep you from getting sick with flu.
- Flu vaccination can reduce the risk of flu-associated hospitalization, including among children and older adults.
- A 2014 study* showed that flu vaccine reduced children’s risk of flu-related pediatric intensive care unit (PICU) admission by 74% during flu seasons from 2010-2012.
- Another study published in the summer of 2016 showed that people 50 years and older who got a flu vaccine reduced their risk of getting hospitalized from flu by 57%.
- Flu vaccination is an important preventive tool for people with chronic health conditions.
- Vaccination was associated with lower rates of some cardiac events among people with heart disease, especially among those who had had a cardiac event in the past year.
- Flu vaccination also has been shown to be associated with reduced hospitalizations among people with diabetes (79%) and chronic lung disease (52%).
- Vaccination helps protect women during and after pregnancy. Getting vaccinated can also protect a baby after birth from flu. (Mom passes antibodies onto the developing baby during her pregnancy.)
- A study that looked at flu vaccine effectiveness in pregnant women found that vaccination reduced the risk of flu-associated acute respiratory infection by about one half.
- There are studies that show that flu vaccine in a pregnant woman can reduce the risk of flu illness in her baby by up to half. This protective benefit was observed for up to four months after birth.
- Flu vaccination also may make your illness milder if you do get sick.
- Getting vaccinated yourself also protects people around you, including those who are more vulnerable to serious flu illness, like babies and young children, older people, and people with certain chronic health conditions.
*References for the studies listed above can be found at Publications on Influenza Vaccine Benefits. Also see the What are the Benefits of Flu Vaccination? [237 KB, 2 pages] fact sheet.
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. These non-flu viruses include rhinovirus (one cause of the "common cold") and respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children, as well as a leading cause of death from respiratory illness in those aged 65 years and older.
Does flu vaccine effectiveness vary by type or subtype?
Yes. VE can vary based on match by type or subtype, but even when recommended vaccine viruses and circulating influenza viruses are alike (well-matched), vaccine effectiveness (VE) may vary depending on virus type or subtype. Since 2009, VE studies have suggested that when vaccine viruses and circulating flu viruses are well-matched, flu vaccines provide better protection against influenza B or influenza A H1N1 viruses than against influenza A H3N2 viruses. A study [505 KB, 10 pages] that looked at pooled VE estimates from 2004-2015 found VE of 33% (CI = 26%–39%) against H3N2 viruses, compared with 61% (CI = 57%–65%) against H1N1 and 54% (CI = 46%–61%) against influenza B viruses. VE estimates were lower when vaccine viruses and circulating viruses were different (not well-matched). The same study found pooled VE of 23% (95% CI: 2% to 40%) against H3N2 viruses when circulating viruses were significantly different from (not well-matched to) the recommended influenza A H3N2 vaccine component.
Why is flu vaccine typically less effective against influenza A H3N2 viruses?
There are a number of reasons why vaccine effectiveness against influenza A H3N2 viruses may be lower.
- While all influenza viruses undergo frequent genetic changes, H3N2 viruses undergo more frequent antigenic changes compared with H1N1 and influenza B viruses. That means that between the time when the composition of the vaccine is recommended and when vaccine is delivered, H3N2 viruses are more likely than H1N1 or influenza B viruses to have changed in ways that could impact how well the vaccine works.
- Growth in eggs is part of the production process for most seasonal influenza vaccines. While all influenza viruses undergo changes when they are grown in eggs, changes in influenza A H3N2 viruses tend to be more varied and more likely to have antigenic implications compared with changes in other influenza viruses. These so-called "egg-adapted changes"are present in the vaccine virus that is recommended for use in vaccine production and may reduce its potential effectiveness against circulating viruses. Other vaccine production technologies, e.g., cell-based vaccine production or recombinant flu vaccines, could circumvent this shortcoming associated with the use of egg-based candidate vaccine viruses in egg-based production technology, but CDC also is using advanced molecular techniques to try to get around this short-coming.
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.
Despite the fact that flu vaccines can work less well in people who are 65 and older, there are many reasons why people in that age group should be vaccinated each year.
- First, people 65 and older are at high risk of getting seriously ill, being hospitalized and dying from the flu.
- Second, while the effectiveness of the flu vaccine can be lower among older people, there are seasons when significant benefit can be observed in terms of averting illness that results in a doctor’s visit. Even if the vaccine provides less protection in older adults than it might in younger people, some protection is better than no protection at all, especially in this high risk group.
- Third, current CDC studies look at how well the vaccine works in preventing flu illness that results in a doctor’s visit or admission to a hospital. This is just one outcome. There are other studies that look at the effects of flu vaccination on hospitalization rates as well as looking at death as on outcome. For example, one study concluded that one death was prevented for every 4,000 people vaccinated against the flu (Fireman et al, 2009).
- In frail elderly adults, hospitalizations can mark the beginning of a significant decline in overall health and mobility, potentially resulting in loss of the ability to live independently or to complete basic activities of daily living. While the protection elderly adults obtain from flu vaccination can vary significantly, a yearly flu vaccination is still the best protection currently available against the flu.
- There are limited data to suggest that flu vaccination may reduce flu illness severity; so while someone who is vaccinated may still get infected, their illness may be milder.
- Fourth, it’s important to remember that people who are 65 and older are a diverse group and often are different from one another in terms of their overall health, level of activity and mobility, and behavior when it comes to seeking medical care. This group includes people who are healthy and active and have responsive immune systems, as well as those who have underlying medical conditions that may weaken their immune system, and therefore, their bodies’ ability to respond to vaccination. Therefore, when evaluating the benefits of flu vaccination, it’s important to look at a broader picture than what one study’s findings can present. Although flu vaccine is not perfect, the overall evidence supports the public health benefit of flu vaccination. Vaccination is particularly important for people 65 and older who are especially vulnerable to serious illness and death, despite the fact that the vaccine may not work as well in this age group.
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.
Some studies do suggest that flu vaccine effectiveness may be higher in people receiving flu vaccine for the first time compared to people vaccinated more than once; other studies have found no evidence that repeat vaccination results in a person being less-protected against flu. Most results from studies assessing the effect of repeat vaccination show that people who do not receive a flu vaccination for the current or previous season are at a higher risk of medical visits due to infection with seasonal flu viruses. Information regarding vaccination history is particularly important to these types of evaluations, and can be difficult to confirm, as accurate vaccination records are not always readily available. People who choose to get vaccinated every year may have different characteristics and susceptibility to flu compared to those who do not seek vaccination every year. CDC thinks that these findings merit further investigation to understand the immune response to repeat vaccination and continued efforts to monitor the effects of repeat vaccination each year. However, based on the substantial burden of flu in the United States, and on the fact that most studies point to vaccination benefits, CDC concludes that yearly flu vaccination remains the first and most important step in protecting against flu and its complications.
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 50-60% 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.
- Page last reviewed: February 15, 2017
- Page last updated: February 15, 2017
- Content source:
- Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)
- Page maintained by: Office of the Associate Director for Communication, Digital Media Branch, Division of Public Affairs