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How The Flu Shot Works (And Why You Should Get It)

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Centers for Disease Control and Prevention

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All data are preliminary and may change as more reports are received. A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page. Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

The percent of specimens testing positive for influenza remains stable, indicating that influenza virus circulation has remained at similar levels during the past two weeks, even while overall levels of respiratory illness have declined.

All data are preliminary and may change as more reports are received. A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page. Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.
Key Points

The percent of specimens testing positive for influenza remains stable, indicating that influenza virus circulation has remained at similar levels during the past two weeks, even while overall levels of respiratory illness have declined. While influenza activity is difficult to predict, it is expected to continue for several more weeks.

The majority of influenza viruses detected are A(H3N2). Most of the H3N2 viruses identified so far this season are genetically closely related to the vaccine virus, but there are some antigenic differences that have developed as H3N2 viruses have continued to evolve.

The percentage of outpatient visits due to respiratory illness decreased nationally again this week but remains above baseline. Influenza is contributing to levels of respiratory illness, but other respiratory viruses are also circulating. The relative contribution of influenza varies by location.

The number of hospital admissions reported to HHS Protect declined slightly again this week.

The cumulative hospitalization rate in the FluSurv-NET system is higher than the rate for the entire 2020-2021 season, but lower than the rate seen at this time during the four seasons preceding the COVID-19 pandemic.

CDC estimates that so far this season there have been at least 2 million flu illnesses, 20,000 hospitalizations, and 1,200 deaths from flu.

There's still time to get vaccinated. An annual flu vaccine is the best way to protect against flu and its potentially serious complications. CDC recommends everyone 6 months and older get a flu vaccine.

Flu vaccination coverage so far is lower this season compared to last.

Flu vaccines are available at many different locations, including pharmacies and health departments. Visit www.vaccines.gov to find a flu vaccine near you.

There are also flu antiviral drugs that can be used to treat flu illness.

Influenza A(H3N2) viruses have been the most frequently detected influenza viruses this season. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent live attenuated influenza vaccine (LAIV) receipt or found upon further testing to be a vaccine virus are not included as they are not circulating influenza viruses.

Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza) are used to monitor whether influenza activity is increasing or decreasing.

Week 3

Data Cumulative since
October 3, 2021
(Week 40)

No. of specimens tested
79,667

1,360,940

No. of positive specimens (%)
1,543 (1.9%)

35,913 (2.6%)

Positive specimens by type

Influenza A
1,503 (97.4%)

35,104 (97.7%)

Influenza B
40 (2.6%)

809 (2.3%)

View Chart Data | View Full Screen

Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating viruses that belong to each influenza subtype/lineage.

Week 3

Data Cumulative since
October 3, 2021
(Week 40)
Positive specimens by type/subtype

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories using the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 460 influenza viruses collected since October 3, 2021. Most of the H3N2 viruses so far are genetically closely related to the vaccine virus, but there are some antigenic differences that have developed as H3N2 viruses have continued to evolve. Virus antigenic data will be reported later this season when a sufficient number of specimens have been tested.

Virus Subtype or Lineage

Genetic Characterization

Total No. of
Subtype/Lineage
Tested

HA
Clade

Number (% of
subtype/lineage
tested)

HA
Subclade

Number (% of
subtype/lineage
tested)

6B.1A

3 (100%)

5a.1

2 (66.7%)

5a.2

1 (33.3%)

3C.2a 1b

437(100%)

1a

1 (0.2%)

3C.3a

0

3a

0

V1A

20 (100%)

V1A

0

Y3

0

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the United States since October 3, 2021, were tested for antiviral susceptibility as follows:

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented.

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with any pathogen that can present with similar symptoms, including influenza, SARS-CoV-2, and RSV. Due to the COVID-19 pandemic, health care-seeking behaviors have changed, and people may be accessing the health care system in alternative settings not captured as a part of ILINet or at a different point in their illness than they might have before the pandemic. Therefore, it is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is tracking the COVID-19 pandemic in a weekly publication called COVID Data Tracker Weekly Review. Information about other respiratory virus activity can be found on CDC's National Respiratory and Enteric Virus Surveillance System (NREVSS) website.

Outpatient Respiratory Illness Visits

Nationwide, during week 3, 2.8% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This percentage is above the national baseline. Five of the 10 HHS regions are above their region-specific baselines (Regions 3,4,6,7, and 10), and the remaining regions are below their baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

* Effective October 3, 2021 (week 40), the ILI definition (fever plus cough or sore throat) no longer includes "without a known cause other than influenza."

View Chart Data (current season only) | View Full Screen

Outpatient Respiratory Illness Visits by Age Group

More than 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet are trending downward for all age groups (0-4 years, 5-24 years, 25-49 years, 50-64 years, and 65+ years).

* Effective October 3, 2021 (week 40), the ILI definition (fever plus cough or sore throat) no longer includes "without a known cause other than influenza."

View Chart Data | View Full Screen

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Activity Level

Number of Jurisdictions

Number of CBSAs

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 1,250 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2021, and January 22, 2022. The overall cumulative hospitalization rate is 4.3 per 100,000 population. This cumulative hospitalization rate is higher than the cumulative in-season hospitalization rate observed for week 3 during the 2020-2021 season (0.5 per 100,000), but lower than the in-season rates observed for week 3 during the 4 seasons preceding the COVID-19 pandemic (these ranged from 14.8 to 41.9 per 100,000 during the 2016-17 through 2019-20 seasons).

When examining rates by age, the highest rate of hospitalization per 100,000 population was among adults aged ≥65 years (11.3); within this group, rates were highest among adults aged ≥85 years (24.0). Among persons aged <65 years, hospitalization rates per 100,000 population were highest among children aged 0-4 years (6.2) followed by adults aged 50-64 years (4.0). When examining rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic American Indian or Alaska Native persons (5.4) followed by non-Hispanic Black persons (5.0).

Among 1,250 hospitalizations, 1,178 (94.2%) were associated with influenza A virus, 62 (5.0%) with influenza B virus, 5 (0.4%) with influenza A virus and influenza B virus co-infection, and 5 (0.4%) with influenza virus for which the type was not determined. Among 295 hospitalizations with influenza A subtype information, 292 (99.0%) were A(H3N2) and 3 (1.0%) were A(H1N1)pdm09. Based on preliminary data, of the 1,250 laboratory-confirmed influenza-associated hospitalizations, 1.9% also tested positive for SARS-CoV-2.

Among 444 hospitalized adults with information on underlying medical conditions, 89.6% had at least one reported underlying medical condition. Among 52 hospitalized children with information on underlying medical conditions, 55.8% had at least one reported underlying medical condition; the most commonly reported was asthma.

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Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics

FluSurv-Net data are used to generate national estimates of the total numbers of influenza cases, medical visits, hospitalizations, and deaths. This season, CDC is reporting preliminary cumulative in-season estimates, which are available at https://www.cdc.gov/flu/about/burden/preliminary-in-season-estimates.htm

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on January 27, 2022, 28.4% of the deaths that occurred during the week ending January 22, 2022 (week 3), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage is above the epidemic threshold of 7.1% for this week. Among the 5,940 PIC deaths reported for this week, 5,160 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and 27 listed influenza, indicating that current PIC mortality is due primarily to COVID-19 and not influenza. The data presented are preliminary and may change as more data are received and processed.

View Chart Dataexcel icon | View Full Screen

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

Influenza-Associated Pediatric Mortality

No influenza-associated pediatric deaths were reported to CDC during week 3.

A total of five influenza-associated pediatric deaths occurring during the 2021-2022 season have been reported to CDC.

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Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.

U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.

World Health Organization:
Additional influenza surveillance information from participating WHO member nations is available through
FluNetexternal icon and the Global Epidemiology Reports.external icon

WHO Collaborating Centers for Influenza:
Australiaexternal icon , Chinaexternal icon , Japanexternal icon , the United Kingdomexternal icon , and the United States (CDC in Atlanta, Georgia)

Europe:
The most up-to-date influenza information from Europe is available from WHO/Europe and the European Centre for Disease Prevention and Controlexternal icon .

Public Health Agency of Canada:
The most up-to-date influenza information from Canada is available in Canada's weekly FluWatch reportexternal icon .

Public Health England:
The most up-to-date influenza information from the United Kingdom is available from Public Health Englandexternal icon .
Any links provided to non-Federal organizations are provided solely as a service to our users. These links do not constitute an endorsement of these organizations or their programs by CDC or the Federal Government, and none should be inferred. CDC is not responsible for the content of the individual organization web pages found at these links.
A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

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Weekly U.S. Influenza Surveillance Report

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All data are preliminary and may change as more reports are received. A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page. Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

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Overview

Flu, also called influenza, is an infection of the nose, throat and lungs, which are part of the respiratory system. The flu is caused by a virus. Influenza is commonly called the flu, but it's different from the stomach "flu" viruses that cause diarrhea and vomiting.

Most people with the flu get better on their own.

But sometimes, influenza and its complications can be deadly. Some groups of people are at a higher than average risk of having flu complications, including:

Young children, especially those age 12 months or younger.

People who plan to be pregnant, are pregnant or recently gave birth during flu season.

Adults older than age 65.

People who live or work in facilities with many other residents. Examples are those in nursing homes and military barracks, as well as people who are in the hospital.

And in the United States, some groups have a higher risk of needing treatment in the hospital for flu. Those groups include people who are American Indian or Alaska Native, Black or Latino.

Other groups at high risk for flu complications include people with:

Weakened immune systems.

A body mass index (BMI) of 40 or higher.

Nervous system disorders or conditions that change how the brain processes information.

And people with certain medical conditions have a higher risk of flu complications, such as:

People who have chronic illnesses, such as asthma, heart disease, kidney disease, liver disease and diabetes.

People who have had strokes.

People who are younger than 20 years of age and receiving long-term aspirin therapy.

Although the annual influenza vaccine isn't 100% effective, it lowers the chances of having severe complications from the flu. This is especially true for people who are at high risk for flu complications.

Symptoms

At first, the flu may seem like a cold with a runny nose, sneezing and sore throat. Colds usually start slowly. But the flu tends to come on quickly. And while a cold can be miserable, you usually feel much worse with the flu.

Common symptoms of the flu often - but not always - include a fever as well as aching muscles, chills and sweats.

Other symptoms include:

Headache.

Dry, persistent cough.

Shortness of breath.

Tiredness and weakness.

Runny or stuffy nose.

Sore throat.

Eye pain.

Vomiting and diarrhea also are flu symptoms. But they are more common in children than in adults.

When to see a doctor

Most people who get the flu can treat themselves at home and often don't need to see a healthcare professional.

If you have flu symptoms and are at risk of complications, see your healthcare professional right away. Taking antiviral medicine to treat the flu may shorten the length of your illness and help prevent more-serious problems.

If you have emergency symptoms of the flu, get medical care right away. For adults, emergency symptoms can include:

Difficulty breathing or shortness of breath.

Chest pain.

Ongoing dizziness.

Seizures.

Worsening of existing medical conditions.

Severe weakness or muscle pain.

Emergency symptoms in children include all the symptoms seen in adults, as well as:

Gray or blue, lips or nail beds.

Dehydration.

Causes

Influenza is caused by viruses. These viruses travel through the air in droplets when someone with the infection coughs, sneezes or talks. You can inhale the droplets directly. Or you can pick up the germs from touching an object, such as a computer keyboard, and then touching your eyes, nose or mouth.

People with the virus are likely contagious from about a day before symptoms appear until about 5 to 7 days after they start. Children and people with weakened immune systems may be contagious for a slightly longer time.

Influenza viruses are constantly changing, with new strains appearing often. If you've had influenza in the past, your body has already made antibodies to fight that specific strain of the virus.

If future influenza viruses are like those you've come across before, either by having the disease or by getting vaccinated, those antibodies may prevent infection or lessen its severity.

But antibody levels may decline over time. And antibodies against past influenza viruses may not protect you from new influenza strains. New strains can be very different from what you had before.

Risk factors

Factors that may raise your risk of getting the flu or its complications include:

Age. Seasonal influenza tends to have worse outcomes in young children, especially those age 12 months and younger. Adults older than age 65 also tend to have worse outcomes.

Living or working conditions. People who live or work in facilities with many other residents, such as nursing homes, are more likely to get the flu. People who are staying in the hospital also are at higher risk.

Weakened immune system. Cancer treatments, anti-rejection medicines, long-term use of steroids, organ transplant, blood cancer or HIV/AIDS can weaken the immune system. This can make it easier to catch the flu virus and may increase the risk of developing complications.

Chronic illnesses. Chronic conditions may increase the risk of influenza complications. Examples include asthma and other lung diseases, diabetes, heart disease, nervous system diseases, previous history of stroke, metabolic disorders, problems with the airway, and kidney, liver or blood disease.

Race or ethnicity. In the United States, people who are American Indian or Alaska Native, Black, or Latino all may have a higher risk of influenza complications.

Aspirin use under age 20. People who are younger than 20 years of age and receiving long-term aspirin therapy are at risk of developing Reye's syndrome if infected with the influenza virus.

Pregnancy. Pregnant people are more likely to develop influenza complications, particularly in the second and third trimesters. This risk continues up to two weeks after the baby is born.

Obesity. People with a body mass index (BMI) of 40 or higher have an increased risk of flu complications.

Complications

If you're young and healthy, the flu usually isn't serious. Although you may feel awful while you have it, the flu usually goes away in a week or two with no lasting effects. But children and adults at high risk may develop complications that may include:

Pneumonia.

Bronchitis.

Asthma flare-ups.

Heart problems.

Ear infections.

Acute respiratory distress syndrome.

Pneumonia is one of the most serious complications. For older adults and people with chronic illnesses, pneumonia can be deadly.

Prevention

Learn more about how to get your flu shot at Mayo Clinic

The U.S. Centers for Disease Control and Prevention (CDC) recommends annual flu vaccination for everyone age 6 months or older. The flu vaccine can lower your risk of getting the flu. The flu vaccine lowers your risk of having serious illness from the flu and needing to stay in the hospital due to flu. The flu vaccine also lowers your risk of dying of flu.

Flu vaccination is especially important because the flu and coronavirus disease 2019 (COVID-19) cause similar symptoms. Both COVID-19 and the flu may be spreading at the same time. Vaccination is the best way to protect against both.

And if a COVID-19 vaccine or booster and a flu vaccination are due at the same time, you can often get vaccinated for both in one visit.

This year's seasonal flu vaccines each provide protection against the four influenza viruses expected to be the most common during this flu season. This year, the vaccine will be available as an injection and as a nasal spray. There also will be high-dose flu vaccines offered for adults age 65 and older.

The nasal spray is approved for people between ages 2 and 49 years old. It isn't recommended for some groups, such as:

People who had a severe allergic reaction to a flu vaccine in the past.

Pregnant people.

Children age 17 years or younger who are taking aspirin or a salicylate-containing medicine.

People with weakened immune systems and caregivers or close contacts of people with weakened immune systems.

Children between ages 2 and 4 years old diagnosed with asthma or wheezing in the past 12 months.

People who recently took antiviral medicine for the flu.

People with a cerebrospinal fluid leak or the potential for a leak, as with a cochlear implant.

If you have an egg allergy, you can still get a flu vaccine.

Controlling the spread of infection

The influenza vaccine isn't 100% effective, so it's also important to take several measures to lower the spread of infection, including:

Wash your hands. Wash your hands well and often with soap and water for at least 20 seconds. If soap and water aren't available, use an alcohol-based hand sanitizer with at least 60% alcohol. Make sure friends and family that you're around regularly, especially kids, know the importance of hand-washing.

Avoid touching your face. Keeping your hands away from your eyes, nose and mouth helps keep germs away from those places.

Cover your coughs and sneezes. Cough or sneeze into a tissue or your elbow. Then wash your hands.

Clean surfaces. Regularly clean often-touched surfaces to prevent spread of infection from touching a surface with the virus on it and then your face.

Avoid crowds. The flu spreads easily wherever people gather - in child care centers, schools, office buildings and auditoriums and on public transportation. By avoiding crowds during peak flu season, you lower your chances of infection.

Also avoid anyone who is sick. And if you're sick, stay home for at least 24 hours after your fever is gone so that you lessen your chance of infecting others.

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Learn more about the symptoms, causes and prevention of this potentially fatal viral infection that attacks the respiratory system.