Vaccine Effectiveness: How Well Do Flu Vaccines Work?
Questions & Answers
- How effective are flu vaccines?
- What are factors that influence how well flu vaccines work?
- What are the benefits of flu vaccination?
- Are flu vaccines effective against all types of flu and cold viruses?
- Does flu vaccine effectiveness vary by type or subtype?
- Why is flu vaccine typically less effective against influenza A(H3N2) viruses?
- How effective are flu vaccines in older adults?
- If older people have weaker immune responses to flu vaccination, should they still get vaccinated?
- How effective are flu vaccines in children?
- Is it true that getting vaccinated repeatedly can reduce vaccine effectiveness?
- What networks does CDC use to study how well flu vaccines work?
- Where can I get more information?
How effective are flu vaccines?
CDC conducts studies each year to determine how well influenza (flu) vaccines protect against flu. While vaccine effectiveness (VE) can vary, recent studies show that flu vaccination reduces the risk of flu illness by between 40% and 60% among the overall population during seasons when most circulating flu viruses are well-matched to those used to make flu vaccines. In general, current flu vaccines tend to work better against influenza B and influenza A(H1N1) viruses and offer less protection against influenza A(H3N2) viruses. More information is available at “Does flu vaccine effectiveness vary by type or subtype?” and “Why is flu vaccine typically less effective against influenza A H3N2 viruses?”.
What are factors that influence how well flu vaccines work?
How well flu vaccines work (or their ability to protect against a certain outcome) can vary from season to season. Protection can vary depending on who is being vaccinated. At least two factors play an important role in determining the likelihood that vaccination will protect a person from flu illness: 1) characteristics of the person being vaccinated (such as their age and health), and 2) how well the vaccines “match” the flu viruses spreading in the community. When flu vaccines are not well matched to some viruses spreading in the community, vaccination may provide little or no protection against illness caused by those viruses. However, the vaccines may still provide protection against other flu viruses that circulate during the season. When there is a good match between flu vaccines and circulating viruses, vaccination provides substantial benefits by preventing flu illness and complications.
Each flu season, researchers try to determine how well flu vaccines work to prevent flu. Estimates of how well a flu vaccine works can vary based on study design, outcome(s) measured, population studied, and type of flu vaccine. Differences between studies must be considered when results are compared. More information is available at How Flu Vaccine Effectiveness and Efficacy are Measured.
What are the benefits of flu vaccination?
Below is a summary of the benefits of flu vaccination and selected scientific studies that support these benefits.
- Flu vaccination can keep you from getting sick with flu.
- Flu vaccine prevents millions of illnesses and flu-related doctor’s visits each year. For example, during 2019-2020, the last flu season prior to the COVID-19 pandemic, flu vaccination prevented an estimated 7.5 million influenza illnesses, 3.7 million influenza-associated medical visits, 105,000 influenza-associated hospitalizations, and 6,300 influenza-associated deaths.
- During seasons when flu vaccine viruses are similar to circulating flu viruses, flu vaccine has been shown to reduce the risk of having to go to the doctor with flu by 40% to 60%.
- Flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick.
- A 2021 study showed that among adults hospitalized with flu, vaccinated patients had a 26% lower risk of intensive care unit (ICU) admission and a 31% lower risk of death from flu compared with those who were unvaccinated.
- A 2018 study showed that among adults hospitalized with flu, vaccinated patients were 59% less likely to be admitted to the ICU than those who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent four fewer days in the hospital than those who were not vaccinated.
- Flu vaccination can reduce the risk of flu-associated hospitalization.
- Flu vaccine prevents tens of thousands of hospitalizations each year. For example, during 2019-2020 flu vaccination prevented an estimated 105,000 flu-related hospitalizations.
- A 2018 study showed that from 2012 to 2015, flu vaccination among adults reduced the risk of being admitted to an ICU with flu by 82%.
- A 2017 study found that during 2009-2016, flu vaccines reduced the risk of flu-associated hospitalization among older adults by about 40% on average.
- A 2014 study showed that flu vaccination reduced children’s risk of flu-related pediatric intensive care unit (PICU) admission by 74% during flu seasons from 2010-2012.
- Flu vaccination is an important preventive tool for people with certain chronic health conditions.
- Flu vaccination has been associated with lower rates of some cardiac events among people with heart disease, especially among those who have had a cardiac event in the past year.
- Flu vaccination can reduce the risk of a flu-related worsening of chronic lung disease (for example, chronic obstructive pulmonary disease (COPD) requiring hospitalization).
- Among people with diabetes and chronic lung disease,flu vaccination has been shown in separate studies to be associated with reduced hospitalizations from a worsening of their chronic condition.
- Flu vaccination during pregnancy helps protect pregnant people from flu during and after pregnancy and helps protect their infants from flu in their first few months of life.
- A 2013 study showed that during the 2010–2011 and 2011–2012 flu seasons vaccination reduced the risk of flu-associated acute respiratory infection in pregnant people by about one-half.
- A 2018 study showed that getting a flu shot reduced a pregnant person’s risk of being hospitalized with flu by an average of 40% from 2010-2016.
- A number of studies have shown that in addition to helping to protect pregnant people from flu, a flu vaccine given during pregnancy helps protect the baby from flu for several months after birth, when babies are too young to be vaccinated.
- Flu vaccine can be lifesaving in children.
- A 2022 study showed that flu vaccination reduced children’s risk of severe life-threatening influenza by 75%.
- A 2020 study found that during the 2018-2019 flu season, flu vaccination reduced flu-related hospitalization by 41% and flu-related emergency department visits by half among children (aged 6 months to 17 years old).
- A 2017 study was the first of its kind to show that flu vaccination can significantly reduce children’s risk of dying from flu.
- Getting vaccinated yourself may also protect 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.
Despite the many benefits offered by flu vaccination, only about half of Americans get an annual flu vaccine. During an average flu season, flu can cause millions of illnesses, hundreds of thousands of hospitalizations and tens of thousands of deaths. Many more people could be protected from flu if more people got vaccinated.
*References for the studies listed above can be found at Publications on Influenza Vaccine Benefits.
Are flu vaccines effective against all types of flu and cold viruses?
Seasonal flu vaccines are designed to protect against infection and illness caused by the flu viruses that research indicates will be most common during the upcoming flu season. Flu vaccines do not protect against infection and illness caused by other viruses that also can 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, but are not limited to, the following:
- SARS-CoV-2 (the virus that causes COVID-19),
- 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 and a major cause of severe respiratory illness in adults 65 years and older.
Does flu vaccine effectiveness vary by type or subtype?
Yes. The amount of protection provided by a flu vaccine may vary by flu virus type or subtype and by vaccine, even when the viruses used to make flu vaccines are similar (or ‘well matched’) to the influenza viruses that are causing illness that season. Since 2009, VE studies looking at how well flu vaccines protect against medically attended illnesses have suggested better protection against influenza B or influenza A(H1N1) viruses than against influenza A(H3N2) viruses. A metanalysis of 76 published VE studies conducted in countries in both the Northern and Southern Hemispheres after the 2009-2010 flu pandemic found flu vaccines performed the best against influenza A(H1N1)pdm09 viruses followed by influenza B viruses. In contrast, flu vaccines were least effective against influenza A(H3N2) viruses. Across all the studies included in the meta-analysis, the pooled VE estimate against all influenza viruses for the Northern Hemisphere was 37 percent. For influenza A(H1N1)pdm09 viruses, the pooled VE estimate for the Northern Hemisphere was 56 percent. For influenza A(H3N2) viruses, the pooled VE estimate for the Northern Hemisphere was 22 percent. And lastly, for influenza B viruses, the pooled VE estimate for studies from the Northern Hemisphere was 42 percent. Overall, VE estimates were lower when the viruses used to make flu vaccines were antigenically different (not well matched) to flu viruses causing illness that season, and VE also appeared to decline with the age of the people studied.
Why is flu vaccine typically less effective against influenza A(H3N2) viruses?
There are several reasons why flu vaccine effectiveness against influenza A(H3N2) viruses may be lower compared to other flu viruses.
- While all flu viruses undergo frequent genetic changes over time, the changes that have occurred in influenza A(H3N2) viruses have more frequently resulted in differences between the virus components of the flu vaccine and circulating influenza viruses (i.e., antigenic changes) compared with influenza A(H1N1) and influenza B viruses. That means that between the time when flu viruses are selected to begin producing vaccines and when flu vaccines are delivered, A(H3N2) viruses are more likely than A(H1N1) or influenza B viruses to have changed in ways that could impact how well the flu vaccines work.
- Growth of viruses in eggs is part of the production process for many seasonal flu vaccines. While all influenza viruses undergo changes when they are grown in eggs, changes in influenza A(H3N2) viruses tend to be more likely to result in antigenic changes compared with changes in other influenza viruses. These so-called “egg-adapted changes” are present in vaccine viruses recommended for use in vaccine production and may reduce their potential effectiveness against circulating influenza viruses. Other vaccine production technologies, e.g., cell-based vaccine production or recombinant flu vaccines, do not use eggs in vaccine production to avoid egg-adapted changes to the viruses used to make vaccines. CDC also is using advanced molecular techniques to improve flu vaccines.
How effective are flu vaccines in older adults?
In numerous studies since 2010, standard dose flu vaccines offered some protection in adults 65 and older against flu illness resulting in a doctor’s visit as well as flu-related hospitalization, but protection was inconsistent in this age group during some flu seasons (1-3). As a result, beginning with the 2022-2023 flu season, CDC and the Advisory Committee on Immunization Practices (ACIP) preferentially recommended the use of higher dose and adjuvanted flu vaccines for people 65 years and older. The preference applies to Fluzone High-Dose Quadrivalent vaccine, Flublok Quadrivalent recombinant flu vaccine and Fluad Quadrivalent adjuvanted flu vaccine. This recommendation was based on a review of available studies which suggests that, in this age group, these vaccines are potentially more effective than standard dose unadjuvanted flu vaccines. Because flu viruses and the effectiveness of flu vaccines can vary from one season to another, it is not known whether, in any given flu season, if one of these preferentially recommended flu vaccines will always be more effective in people 65 and older.
If older people have weaker immune responses to flu vaccination, should they still get vaccinated?
There are many reasons why people in this age group should be vaccinated each year:
- First, people 65 years and older are at increased risk of serious illness, hospitalization, and death from flu.
- Second, while the effectiveness of flu vaccines can be lower among some older people (particularly against influenza A(H3N2) viruses), there are seasons when significant benefit can be observed (this is particularly true against influenza A(H1N1) and influenza B viruses).
- Third, flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick.
- Flu vaccines can protect against more serious outcomes like hospitalization and death. For example, one study concluded that one death was prevented for every 4,000 people vaccinated against flu.
- Fourth, in adults 65 years and older, 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 older adults obtain from flu vaccination can vary significantly, a yearly flu vaccination is still the best protection currently available against flu.
- In addition, people who are 65 and older differ in overall health, level of activity and mobility, and health care seeking behavior. 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 systems and their bodies’ ability to respond to vaccination. Therefore, when evaluating the benefits of flu vaccination, it is important to look at a broader picture than what one study’s findings can present.
How effective are flu vaccines in children?
Flu vaccination has been found during most seasons to provide a similar level of protection against flu illness in children to that seen among adults. ACIP recommends any licensed, age-appropriate flu vaccine for children without preference for any one flu vaccine over another. In several studies, flu vaccine effectiveness was higher among children who received two doses of flu vaccine the first season that they were vaccinated (as recommended) compared to “partially vaccinated” children who only received a single dose of flu vaccine (4-6).
In addition to preventing illness, flu vaccine can prevent severe, life-threatening complications in children, for example:
- A 2022 study showed that flu vaccination reduced children’s risk of severe life-threatening influenza by 75 percent.
- A 2020 study found that during the 2018-2019 flu season, flu vaccination reduced flu-related hospitalization by 41 percent and flu-related emergency department visits by half among children (aged 6 months to 17 years old).
- In 2017, a study in the journal Pediatrics was the first of its kind to show that flu vaccination also significantly reduced a child’s risk of dying from flu. The study, which looked at data from four flu seasons between 2010 and 2014, found that flu vaccination reduced the risk of flu-associated death by half (51 percent) among children with underlying, higher risk medical conditions and by nearly two-thirds (65 percent) among healthy children.
- A 2014 study showed that flu vaccine reduced children’s risk of flu-related pediatric intensive care unit (PICU) admission by 74 percent during flu seasons from 2010-2012.
Is it true that getting vaccinated repeatedly can reduce vaccine effectiveness?
Scientists continue to study and learn more about how prior or repeat flu vaccinations affect vaccine effectiveness. Studies have shown that previous flu vaccinations or infections can affect the immune response to flu vaccination in variable ways. A 2021 study conducted over six flu seasons (2012-2013 to 2017-2018) found that among people who get a flu vaccine each year (i.e., repeat vaccinees), getting the current season’s flu vaccine generally was beneficial in reducing flu illness that results in a medical visit. There was no significant loss in protection among study participants who received the current and prior season’s flu vaccine compared to those who were only vaccinated during the current season. Importantly, among people with prior season vaccination, current season vaccination provided significant added protection in some seasons overall and against A(H1N1)pdm09- and influenza B-associated illnesses. These findings support the current ACIP recommendations for annual flu vaccination and demonstrate that study participants vaccinated during the prior flu season benefit overall from getting vaccinated during the current season.
Some studies have shown reduced flu vaccine effectiveness–particularly against A(H3N2) viruses–among people who were vaccinated in both the current and prior season compared with those who had only been vaccinated in the current season. However, this finding could be the result of study bias related to the “carryover” benefits of the prior season’s vaccine. Of note, when the benefits of current flu vaccines are analyzed separately for people only vaccinated this season and people vaccinated both this season and the prior season, benefits of current season flu vaccination are observed for both groups. CDC is learning more about how repeat vaccination affects vaccine effectiveness, but current findings continue to support the Advisory Committee on Immunization Practices’ recommendations for annual flu vaccination.
What networks does CDC use to study how well flu vaccines work?
Information about the networks CDC uses to study flu vaccine effectiveness is available at CDC’s Influenza Vaccine Effectiveness Networks.
Where can I get more information?
- Edward A. Belongia, Danuta M. Skowronski, Huong Q. McLean et al. Repeated annual influenza vaccination and vaccine effectiveness: review of evidence. Expert Review of Vaccines. 2017 Jun; 16(7): 723-36. doi: 10.1080/14760584.2017.1334554.
- Marc Rondy, Nathalie El Omeiri, Mark G. Thompson, et al. Effectiveness of influenza vaccines in preventing severe influenza illness among adults: A systemic review and meta-analysis of test-negative design case-control studies. Journal of Infection. Sept 2017; 65: 381-394. doi: 10.1016/j.jinf.2017.09.010
- Kate Russell, Jessie R. Chung, Arnold S. Monto et al. Influenza vaccine effectiveness in older adults compared with younger adults over five seasons. Vaccine. Feb 2018; 36(10): 1272-1278. doi: 10.1016/j.vaccine.2018.01.045.
- Chung J, Flannery B, Gaglani M et al. Patterns of Influenza Vaccination and vaccine effectiveness among young US Children Who Receive Outpatient Care for Acute Respiratory Tract Illness. JAMA Pediatr. 2020 Jul 1;174(7):705-713. doi: 10.1001/jamapediatrics.2020.0372.
- Thompson M, Clippard J, Petrie J et al. Influenza Vaccine Effectiveness for Fully and Partially Vaccinated Children 6 Months to 8 Years Old During 2011-2012 and 2012-2013: The Importance of Two Priming Doses. Pediatr Infect Dis J. 2016 Mar;35(3):299-308. doi: 10.1097/INF.0000000000001006.
- Feldstein L, Ogokeh C, Rha B et al. Vaccine Effectiveness Against Influenza Hospitalization Among Children in the United States, 2015-2016. Journal of the Pediatric Infectious Diseases Society. 2021 Feb; 10(2):75-82. https://doi.org/10.1093/jpids/piaa017.