Influenza - HAN
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For situational awareness of the ongoing outbreak of highly pathogenic avian influenza see: H5N1 Bird Flu: Current Situation Summary.
See CDC Resources for Health Care Providers for current interim guidance including clinical testing and diagnosis, use of antivirals, and infection prevention guidelines.
See CDC Resources for Health Care Providers for current interim guidance including clinical testing and diagnosis, use of antivirals, and infection prevention guidelines.
Overview
Influenza (also known as “flu”) is a contagious respiratory illness caused by influenza viruses. There are two main types of human flu viruses: Types A and B. Type A viruses are the only influenza viruses known to cause flu pandemics. Influenza A has subtypes determined by the surface antigens hemagglutinin (HA) and neuraminidase (NA). There are 18 different H subtypes and 11 different N subtypes. Type B influenza is classified into two lineages: B/Yamagata and B/Victoria, although there have been no confirmed detections of influenza B/Yamagata lineage viruses after March 2020. Influenza B more commonly affects children.
Flu viruses are constantly changing. One way that flu viruses change is called “antigenic drift,” which consists of small, gradual genetic changes that occur continuously over time as the virus replicates. Flu A viruses can also undergo a more abrupt, major change called “antigenic shift,” which can result in a new flu subtype. This shift can happen if a flu virus from an animal population gains ability to infect humans, for instance.
Clinical Presentation
Flu can cause mild to severe illness, and at times can lead to death. Flu symptoms usually come on suddenly. Symptoms begin 1–4 days after exposure and usually last 3–7 days, although can persist for >2 weeks especially in the elderly and those with chronic lung conditions. People who have flu often feel some or all of these symptoms:
Influenza Complications
Most people who get flu will recover in a few days to less than two weeks, but some people will develop complications as a result of flu, some of which can be life-threatening and result in death.
Complications include:
Anyone can get sick with flu, even healthy people, and serious problems related to flu can happen to anyone at any age, but some people are at higher risk of developing serious flu-related complications if they get sick.
Higher risk groups include:
Flu viruses are constantly changing. One way that flu viruses change is called “antigenic drift,” which consists of small, gradual genetic changes that occur continuously over time as the virus replicates. Flu A viruses can also undergo a more abrupt, major change called “antigenic shift,” which can result in a new flu subtype. This shift can happen if a flu virus from an animal population gains ability to infect humans, for instance.
Clinical Presentation
Flu can cause mild to severe illness, and at times can lead to death. Flu symptoms usually come on suddenly. Symptoms begin 1–4 days after exposure and usually last 3–7 days, although can persist for >2 weeks especially in the elderly and those with chronic lung conditions. People who have flu often feel some or all of these symptoms:
- fever or feeling feverish/chills
- cough
-
sore throat
-
runny or stuffy nose
-
muscle aches
-
headaches
-
fatigue
-
some people may have vomiting and diarrhea, but this is more common in children than adults.
Influenza Complications
Most people who get flu will recover in a few days to less than two weeks, but some people will develop complications as a result of flu, some of which can be life-threatening and result in death.
Complications include:
- sinus and ear infections
- pneumonia
- worsening of chronic medical problems. For example, people with asthma may experience asthma attacks while they have flu.
- inflammation of the heart (myocarditis), brain (encephalitis) or muscle tissues (myositis, rhabdomyolysis)
- multi-organ failure (for example, respiratory and kidney failure)
- sepsis (an extreme inflammatory response triggered by flu virus)
Anyone can get sick with flu, even healthy people, and serious problems related to flu can happen to anyone at any age, but some people are at higher risk of developing serious flu-related complications if they get sick.
Higher risk groups include:
- people 65 years and older
- people of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease)
- pregnant people
- people living in nursing homes and other long-term care facilities
- children younger than 5 years, but especially those younger than 2 years old
testing
Influenza virus testing is not required to make a clinical diagnosis of influenza in outpatients with suspected influenza, particularly during increased influenza activity when seasonal influenza A and B viruses are circulating in the local community. However, influenza virus testing can inform clinical management when the results may influence clinical decisions such as whether to initiate antiviral treatment, perform other diagnostic testing, or implement infection prevention and control measures for influenza. Influenza virus testing is recommended for all patients with suspected influenza who are being admitted to a hospital and those residing in a long-term care facility. Most importantly, clinicians should understand the limitations of influenza virus tests and how to properly interpret the results, particularly negative results. During a respiratory illness outbreak in a closed setting (e.g., hospitals, long-term care facility, cruise ship, boarding school, summer camp) testing for influenza virus infection can be very helpful in determining if influenza is the cause of the outbreak.
Laboratory Tests
Diagnostic tests available for detection of influenza viruses in respiratory specimens include molecular assays (including rapid molecular assays, reverse transcription polymerase chain reaction (RT-PCR) and other nucleic acid amplification tests); and antigen detection tests (including rapid influenza diagnostic tests and immunofluorescence assays). Viral culture is important for public health purposes but does not provide timely results to inform clinical management. Serological testing does not provide timely results to inform clinical management decisions.
For more information on influenza testing, see this page: https://www.cdc.gov/flu/hcp/testing-methods/?CDC_AAref_Val=https://www.cdc.gov/flu/professionals/diagnosis/overview-testing-methods.htm
Testing At the IDPH Public Health Laboratory (PHL)
The following specimens should be sent to the IDPH PHL for influenza testing and characterization:
Laboratory Tests
Diagnostic tests available for detection of influenza viruses in respiratory specimens include molecular assays (including rapid molecular assays, reverse transcription polymerase chain reaction (RT-PCR) and other nucleic acid amplification tests); and antigen detection tests (including rapid influenza diagnostic tests and immunofluorescence assays). Viral culture is important for public health purposes but does not provide timely results to inform clinical management. Serological testing does not provide timely results to inform clinical management decisions.
For more information on influenza testing, see this page: https://www.cdc.gov/flu/hcp/testing-methods/?CDC_AAref_Val=https://www.cdc.gov/flu/professionals/diagnosis/overview-testing-methods.htm
Testing At the IDPH Public Health Laboratory (PHL)
The following specimens should be sent to the IDPH PHL for influenza testing and characterization:
- Influenza specimens that cannot be subtyped (e.g., from molecular assays that can detect all currently circulating influenza A virus subtypes who identify an unsubtypeable result).
- Specimens that are approved by CDPH on a case-by-case basis, such as for outbreak management or virus identification in a congregate facility, post-mortem evaluation, and cases of suspected novel influenza viruses.
If approved, an authorization code will be provided that will authorize influenza testing at the IDPH Laboratory. Specimens submitted to IDPH Laboratory without prior approval will be rejected and stored until further information is obtained. To request authorization:
- Call 312-743-9000 and press 2 when prompted. If no answer, leave a voicemail with a contact name and a call-back number.
- When contacted by CDPH, please have the patient information readily available and the reason why influenza testing is being requested at the IDPH Laboratory. If approved, an authorization code will be provided during the call.
- Once approved, the provider must submit the test order electronically using the IDPH Electronic Test Ordering and Reporting (ETOR) portal. To enroll email DPH.Labs.DMG@Illinois.gov
- The submitter must arrange for the specimen to be transported to the IDPH Laboratory. All specimens must be received at the IDPH Laboratory for testing within three days of specimen collection except if frozen.
- Specimen results will be communicated to the submitter as reported in the ETOR portal.
For more information about reporting: IDPH Respiratory Testing and Reporting Guidelines.
Treatment
There are prescription influenza antiviral drugs that can be used to treat flu illness. These drugs can lessen symptoms and shorten illness by 1 or 2 days. Antiviral treatment is recommended as soon as possible, ideally within one to two days after flu symptoms begin, for any patient with suspected or confirmed influenza who:
Decisions about starting antiviral treatment for patients with suspected influenza should not wait for laboratory confirmation of influenza virus infection. Empiric antiviral treatment should be started as soon as possible in the above priority groups.
Clinicians can consider early empiric antiviral treatment of non-higher-risk outpatients with suspected influenza based upon clinical judgment if treatment can be initiated within 48 hours of illness onset.
For control of outbreaks in institutional settings (e.g., long-term care facilities for older adults and children) and hospitals, CDC recommends antiviral chemoprophylaxis of exposed residents with oral oseltamivir or inhaled zanamivir for a minimum of 2 weeks and continuing up to 1 week after the last known case was identified. Antiviral chemoprophylaxis is recommended for all residents, including those who have received influenza vaccination. For control of some institutional influenza outbreaks, post-exposure antiviral treatment has been used (e.g., oseltamivir twice daily for 5 days) instead of post-exposure antiviral chemoprophylaxis. Baloxavir is approved for post-exposure prophylaxis (single dose) of influenza in persons aged 5 years and older within 48 hours of contact with an individual with influenza.
For more information about influenza treatments, see this page: https://www.cdc.gov/flu/hcp/antivirals/summary-clinicians.html?CDC_AAref_Val=https://www.cdc.gov/flu/professionals/antivirals/summary-clinicians.htm
- is hospitalized
- is living in a long-term care facility
- has severe, complicated, or progressive illness, or
- is at higher risk for influenza complications
Decisions about starting antiviral treatment for patients with suspected influenza should not wait for laboratory confirmation of influenza virus infection. Empiric antiviral treatment should be started as soon as possible in the above priority groups.
Clinicians can consider early empiric antiviral treatment of non-higher-risk outpatients with suspected influenza based upon clinical judgment if treatment can be initiated within 48 hours of illness onset.
For control of outbreaks in institutional settings (e.g., long-term care facilities for older adults and children) and hospitals, CDC recommends antiviral chemoprophylaxis of exposed residents with oral oseltamivir or inhaled zanamivir for a minimum of 2 weeks and continuing up to 1 week after the last known case was identified. Antiviral chemoprophylaxis is recommended for all residents, including those who have received influenza vaccination. For control of some institutional influenza outbreaks, post-exposure antiviral treatment has been used (e.g., oseltamivir twice daily for 5 days) instead of post-exposure antiviral chemoprophylaxis. Baloxavir is approved for post-exposure prophylaxis (single dose) of influenza in persons aged 5 years and older within 48 hours of contact with an individual with influenza.
For more information about influenza treatments, see this page: https://www.cdc.gov/flu/hcp/antivirals/summary-clinicians.html?CDC_AAref_Val=https://www.cdc.gov/flu/professionals/antivirals/summary-clinicians.htm
Prevention (Vaccination)
Seasonal flu vaccines are designed to protect against the influenza viruses that research indicates will be most common during the upcoming season. Beginning again in 2024-2025, all flu vaccines in the United States will be “trivalent” vaccines, which means they protect against three different flu viruses: an influenza A(H1N1) virus, an influenza A(H3N2) virus, and an influenza B/Victoria virus.
Everyone 6 months and older in the United States, with rare exception, should get an influenza vaccine every season. CDC’s Advisory Committee on Immunization Practices has made this “universal” recommendation since the 2010-2011 influenza season.
For most people who need only one dose of influenza vaccine for the season, September and October are generally good times to be vaccinated against influenza. Ideally, everyone should be vaccinated by the end of October.
For more information about flu vaccines, see this page: https://www.cdc.gov/flu/hcp/vax-summary/?CDC_AAref_Val=https://www.cdc.gov/flu/professionals/vaccination/vax-summary.htm
Everyone 6 months and older in the United States, with rare exception, should get an influenza vaccine every season. CDC’s Advisory Committee on Immunization Practices has made this “universal” recommendation since the 2010-2011 influenza season.
For most people who need only one dose of influenza vaccine for the season, September and October are generally good times to be vaccinated against influenza. Ideally, everyone should be vaccinated by the end of October.
For more information about flu vaccines, see this page: https://www.cdc.gov/flu/hcp/vax-summary/?CDC_AAref_Val=https://www.cdc.gov/flu/professionals/vaccination/vax-summary.htm
Reporting
In Illinois, the following influenza related conditions are reportable:
For more information about reporting see: https://www.ilga.gov/commission/jcar/admincode/077/07700690sections.html
- Suspected novel influenza (e.g., severe respiratory illness of unknown etiology associated with recent international travel, contact with swine, birds, or cattle, or any case of human infection with an influenza A virus that is different from currently circulating human influenza H1 and H3 viruses). Suspected Novel Influenza cases are reportable immediately, within three hours upon initial clinical suspicion. See the Novel and Variant Influenza section for more information.
- Pediatric influenza-associated death is defined as death of an individual < 18 years of age resulting from a clinically compatible illness confirmed to be influenza by culture, PCR, commercial rapid influenza, or other appropriate diagnostic test. These cases are reportable as soon as possible, but within 3 days.
- Influenza-associated Intensive Care Unit (ICU) hospitalizations are defined as individuals hospitalized in an ICU with a positive laboratory test for influenza. These cases are reportable as soon as possible, but within 3 days.
- Outbreaks of respiratory illness in a congregate setting (e.g., correctional or long-term care facility): Additional information regarding reporting of outbreaks of respiratory illness in congregate settings can be found in the Information for Long-Term Care Facilities section. Outbreaks are reportable within 24 hours of identification.
For more information about reporting see: https://www.ilga.gov/commission/jcar/admincode/077/07700690sections.html
Novel and Variant Influenza
Novel influenza A virus infections include all human infections with influenza A viruses that are different from currently circulating human seasonal influenza H1 and H3 viruses. Novel viruses include those that are subtyped as nonhuman in origin and those that are not able to be subtyped using standard laboratory methods and reagents. Novel subtypes include, but are not limited to, H2, H5, H7, and H9 subtypes.
Rapid detection and reporting of human infections with novel influenza A viruses – viruses against which there is little to no pre-existing immunity – is important to facilitate prompt awareness and characterization of influenza A viruses with pandemic potential and accelerate the implementation of effective public health responses.
Avian Influenza A Viruses
Avian influenza A viruses do not normally infect humans, but sporadic human infections have occurred. Illness in humans caused by avian influenza A virus infections has ranged from mild to severe (e.g., pneumonia). Several subtypes of avian influenza A viruses are known to have infected people (H5, H6, H7, H9, H10 viruses). Highly pathogenic Asian avian influenza A(H5N1) and low pathogenic Asian A(H7N9) viruses account for the majority of human infections with avian influenza A viruses. More information about specific novel influenza A viruses, including those that have caused illness in humans, is available.
Highly pathogenic avian influenza (HPAI) A(H5N1) viruses have been detected in U.S. wild birds, commercial poultry, and backyard flocks beginning in January 2022 and in dairy cattle beginning in March 2024. CDC considers the current risk to the general public’s health in the U.S. to be low. However, because the recently detected HPAI A(H5N1) viruses are related to viruses that have caused severe disease in infected humans, they should be regarded as having the potential to cause severe disease in humans until shown otherwise. For more information visit the CDC Avian Influenza Current Situation Summary.
Clinicians should consider the following for surveillance and testing:
Rapid detection and reporting of human infections with novel influenza A viruses – viruses against which there is little to no pre-existing immunity – is important to facilitate prompt awareness and characterization of influenza A viruses with pandemic potential and accelerate the implementation of effective public health responses.
Avian Influenza A Viruses
Avian influenza A viruses do not normally infect humans, but sporadic human infections have occurred. Illness in humans caused by avian influenza A virus infections has ranged from mild to severe (e.g., pneumonia). Several subtypes of avian influenza A viruses are known to have infected people (H5, H6, H7, H9, H10 viruses). Highly pathogenic Asian avian influenza A(H5N1) and low pathogenic Asian A(H7N9) viruses account for the majority of human infections with avian influenza A viruses. More information about specific novel influenza A viruses, including those that have caused illness in humans, is available.
Highly pathogenic avian influenza (HPAI) A(H5N1) viruses have been detected in U.S. wild birds, commercial poultry, and backyard flocks beginning in January 2022 and in dairy cattle beginning in March 2024. CDC considers the current risk to the general public’s health in the U.S. to be low. However, because the recently detected HPAI A(H5N1) viruses are related to viruses that have caused severe disease in infected humans, they should be regarded as having the potential to cause severe disease in humans until shown otherwise. For more information visit the CDC Avian Influenza Current Situation Summary.
Clinicians should consider the following for surveillance and testing:
- Consider the possibility of infection with novel influenza A viruses known to cause severe disease or with the potential to cause severe disease in humans in patients who present with acute respiratory infection (ARI) symptoms or conjunctivitis, and who have had recent direct or close contact (particularly unprotected exposure without use of respiratory protection and eye protection) <10 days prior to illness onset to animals and materials from animals potentially infected or confirmed to be infected with HPAI A(H5N1) virus. A full list of clinical criteria for novel influenza A virus testing is available.
- If infection with a novel influenza A virus with the potential to cause severe disease in humans is suspected, specimens should be collected while following recommended infection control precautions. Notify CDPH as soon as possible by calling 312-743-9000 and press 2 when prompted. After hours, call 311 and ask to speak with the physician on-call. We will work with you to submit specimens to the IDPH public health laboratory for testing.
- Suspected cases of novel influenza should be reported in the Illinois Disease Surveillance System immediately and should not wait until results have been returned.
Variant Influenza
Influenza H1 and H3 subtypes originating from a non-human species or from genetic reassortment between animal and human viruses are also novel viruses. For example, human infections with an influenza virus that normally circulates in swine and not people are called variant viruses. They can be denoted by adding the letter “v” to the end of the virus subtype designation. Human infections with A(H1N1)v, A(H1N2)v, and A(H3N2)v viruses have been detected in the United States (see https://www.cdc.gov/swine-flu/variant-flu-in-humans/risk-and-reporting.html?CDC_AAref_Val=https://www.cdc.gov/flu/swineflu/variant.htm).
Variant virus infection cannot be distinguished by clinical features from seasonal influenza virus infection or from infection with other respiratory viruses that can cause influenza-like illness. Therefore, the key to suspecting variant virus infection in an ill patient is to elicit an epidemiological link to recent swine exposure in the week prior to illness onset. Exposure can be defined as follows:
- Direct contact with swine (e.g., showing swine, raising swine, feeding swine, or cleaning swine waste)
- Indirect exposure to swine (e.g., visiting a swine farm or walking through a swine barn), especially if swine were known to be ill; or
- Close contact (within 2 meters or approximately 6 feet) with an ill person who had recent swine exposure or is known to be infected with a variant virus.
For any ill person with an exposure as defined above, respiratory samples should be taken for testing.
- Contact CDPH as soon as possible by calling 312-743-9000 and press 2 when prompted. After hours, call 311 and ask to speak with the physician on call. We will help arrange for appropriate testing of clinical specimens at the IDPH public health laboratory.
- Suspected cases of variant influenza should be reported in the Illinois Disease Surveillance System immediately and should not wait until results have been returned.
Additional Resources
- Clinician Brief: Evaluating and Managing Patients Exposed to Animals or Persons Infected with Novel Influenza A Viruses of Public Health Concern
- Interim Guidance for Infection Control Within Healthcare Settings When Caring for Confirmed Cases, Probably Cases, and Cases Under Investigation for Infection with Novel Influenza A Viruses Associated with Severe Disease
- Interim Guidance on Specimen Collection and Testing for Patients with Suspected Infection with Novel Influenza A Viruses Associated with Severe Disease or with the Potential to Cause Severe Disease in Humans
- Interim Guidance on the Use of Antiviral Medications for Treatment of Human Infections with Novel Influenza A Viruses Associated with Severe Human Disease
- Interim Guidance for Clinicians on Human Infections with Variant Influenza Viruses
Information for Congregate Living Facilties
Outbreaks of respiratory illness in a congregate setting (e.g., correctional or long-term care facility) are reportable to the Chicago Department of Public Health.
Outbreak definition and key terms
Acute respiratory illness
Acute respiratory illness (ARI) is an illness characterized by any two of the following signs and symptoms that are new or worsening from the resident's normal state:
Outbreak Definition
Outbreaks must be reported if they meet the following criteria:
Acute Respiratory Illness (ARI) or Viral Respiratory Diseases (including SARS-CoV-2, Influenza, Respiratory Syncytial Virus (RSV), Parainfluenza, Human Metapneumovirus, Respiratory Adenovirus, Rhino/Enterovirus, or other viral respiratory diseases meeting the outbreak definition)
Three or more residents and/or staff in a facility with:
Reporting Outbreaks
Online: IDPH Respiratory Illness Outbreak Report Form for Congregate Settings
Note: There may be a need to report cases and/or outbreaks to multiple entities. Reporting to one does NOT satisfy the need to report to the others.
For additional information see: Preventing and Controlling Acute Respiratory Illness Outbreaks in Skilled Nursing Facilities and Other Facilities Providing Skilled Care
Additional Resources
Outbreak definition and key terms
Acute respiratory illness
Acute respiratory illness (ARI) is an illness characterized by any two of the following signs and symptoms that are new or worsening from the resident's normal state:
- Fever (greater than 100°F/37.8°C or more than two degrees above a resident’s established baseline)
- Cough (productive or nonproductive)
- Runny nose or nasal congestion
- Sore throat
- Muscle aches
- Shortness of breath or difficulty breathing, which may manifest as increased fatigue
- Low oxygen saturation in the blood (normal levels are between 95 and 100%, but may vary for people with certain medical conditions)
Outbreak Definition
Outbreaks must be reported if they meet the following criteria:
Acute Respiratory Illness (ARI) or Viral Respiratory Diseases (including SARS-CoV-2, Influenza, Respiratory Syncytial Virus (RSV), Parainfluenza, Human Metapneumovirus, Respiratory Adenovirus, Rhino/Enterovirus, or other viral respiratory diseases meeting the outbreak definition)
Three or more residents and/or staff in a facility with:
- Acute respiratory illness (ARI) and/or
- Positive point-of-care test (as available) and/or
- Laboratory-confirmed viral infection within 72 hours of each other-AND-
- At least one of the cases is a resident.
Reporting Outbreaks
Online: IDPH Respiratory Illness Outbreak Report Form for Congregate Settings
Note: There may be a need to report cases and/or outbreaks to multiple entities. Reporting to one does NOT satisfy the need to report to the others.
For additional information see: Preventing and Controlling Acute Respiratory Illness Outbreaks in Skilled Nursing Facilities and Other Facilities Providing Skilled Care
Additional Resources
- Long-term Care Facilities Chicago HAN page (CDPH)
- Interim Guidance for Influenza Outbreak Management in Long-Term Care and Post-Acute Care Facilities (CDC)
- Preventing and Controlling Acute Respiratory Illness Outbreaks in Skilled Nursing Facilities and Other Facilities Providing Skilled Care (IDPH)
- Testing and Management Considerations for Nursing Home Residents with Acute Respiratory Illness Symptoms when SARS-CoV-2 and Influenza Viruses are Co-circulating (CDC)
- Clinical Practice Guidelines by the Infectious Diseases Society of America: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenza (IDSA)
- Respiratory Illness Prevention in Long-Term Care Facilities Poster (CDPH)
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Additional Resources
Influenza Surveillance and Data
The Chicago Department of Public Health summarizes key respiratory virus surveillance indicators into a weekly report. The indicators are compiled from laboratory-based data as well emergency department visit data. All data are preliminary and may change as additional reports are received.
Data that is displayed on the online influenza dashboard is available for download on the City of Chicago Data Portal.
Influenza Testing
Influenza Vaccine
The Chicago Department of Public Health summarizes key respiratory virus surveillance indicators into a weekly report. The indicators are compiled from laboratory-based data as well emergency department visit data. All data are preliminary and may change as additional reports are received.
Data that is displayed on the online influenza dashboard is available for download on the City of Chicago Data Portal.
Influenza Testing
- IDPH Influenza Virus Specimen Submission Instructions (IDPH)
- Guide for considering influenza testing when influenza viruses are circulating in the community
Influenza Vaccine
- Seasonal Influenza Vaccination Resources for Health Professionals (CDC)
- Talking About Influenza Vaccine Recommendation
- CDPH Community Flu/COVID-19 Clinics
- Outbreaks of influenza in school or childcare settings are not reportable in Illinois.
- See CDPH’s School HAN flu page for more information.
- Preventing Spread of Infections in K-12 Schools (CDC)
Influenza Reporting
For Clinical Questions Contact:
The CDPH Disease Reporting Hotline at 312-743-9000
*After hours, weekends, and holidays, call 311 and ask for the communicable disease physician on-call (or 312-744-5000 if outside the City of Chicago)
Schools: please click here for reporting and management
Reporting Guidelines:
Please refer to the relevant section in the left portion of this page.
