Influenza-associated conditions
At a glance
- Reporting time frame for hospitalizations and deaths: 4 calendar days
- Reporting time frame for outbreaks: Immediately (by phone within 4 hours of suspected diagnosis)
- Individual cases need follow up? No
- CDPHE program: Viral Respiratory Diseases
- Mode(s) of transmission: respiratory droplets
- Incubation period: 1 to 4 days (average of two days)
- Infectious period: One day before symptom onset and 5–7 days after symptom onset. People with flu are most contagious during the first three to four days of their illness. Young children and immunocompromised people may be contagious for longer periods of time.
- Treatment: The majority of people with mild illness caused by influenza will likely not need medical care or treatment. Antiviral medication (e.g., oseltamivir) can be administered to patients at risk or showing signs of severe illness. These medications are most effective when administered early in the course of illness.
- Prophylaxis: During a confirmed influenza outbreak in a long-term or other residential care facility, antiviral chemoprophylaxis is recommended for all medically eligible residents who are not exhibiting influenza-like illness.
- Exclusion criteria: Someone who is confirmed or suspected to have influenza should stay home for at least 24 hours after their fever has resolved (without the use of fever-reducing medications), except to get medical care or other necessities.
- Additional: The Centers for Disease Control and Prevention (CDC) recommends everyone 6 months of age and older receive an annual influenza vaccine.
Contents
Within Colorado, influenza-associated hospitalizations and influenza-associated pediatric deaths (people younger than 18 years old) are reportable. Additionally, positive influenza results are reportable from laboratories that report conditions via electronic lab reporting (ELR). Lastly, all outbreaks are immediately reportable to public health regardless of setting.
Refer to the list of reportable conditions in Colorado to ensure all required conditions are being reported.
What and how to report to the Colorado Department of Public Health and Environment (CDPHE) or local public health agency
- Cases of influenza-associated hospitalization and influenza-associated pediatric death should be reported within four days using the EpiTrax Reportal, electronically via electronic case reporting (eCR) (if hospital has been onboarded via CDPHE eCR team), fax, or telephone to CDPHE or the local public health agency.
- All influenza-positive results submitted via ELR by onboarded laboratories
- Known or suspected outbreaks should be reported immediately (within four hours of detection) by telephone to either the local public health agency or to CDPHE at 303-692-2700 (after hours 303-370-9395). Additional details should be provided by completing the online outbreak information form.
Purpose of surveillance and reporting
- Provide data on influenza severity and burden across Colorado.
- Monitor trends in disease circulation to help inform public health decisions regarding disease prevention.
- Detect novel influenza strains.
- Promptly identify and respond to outbreaks.
Etiologic agent
Influenza has four distinct viral types: influenza A, B, C, and D. Influenza A and B are the most common types and the primary causes of seasonal influenza outbreaks. Influenza A is commonly broken down further into subtypes according to the combination of proteins on the surface of the virus, hemagglutinin (H) and neuraminidase (N). These protein combinations create specific viral strains such as H1N1 and H3N2, which are the current subtypes circulating within humans. Influenza pandemics are only known to be caused by influenza A viruses. Influenza B is classified into lineages such as B/Victoria. Influenza C appears less frequently and causes only mild infection. Influenza D is not known to cause illness in people but causes illness in cattle.
Clinical description
Signs and symptoms
Symptoms of an uncomplicated influenza infection can include fever, chills, cough, sore throat, headache, fatigue, runny nose, and muscle or body aches. Children in particular might also experience vomiting or diarrhea alongside respiratory symptoms. Fever may not present, and the elderly and immunosuppressed in particular may have atypical symptoms. Uncomplicated influenza will usually resolve after 3 to 7 days, but a cough may persist for more than two weeks, particularly if the person has underlying chronic conditions.
Complications
Certain populations have an increased risk of complications when infected with influenza such as people older than 65 years, those with chronic illnesses or immunocompromising conditions, pregnant people, certain people of racial and ethnic minority groups, and children younger than 5 years. Specifically in children, influenza infection can lead to cardiac, musculoskeletal, and neurologic complications. Influenza can result in dehydration for people of any age and aggravate underlying conditions. Pneumonia due to influenza or a secondary bacterial pathogen is another potential complication that can occur.
Reservoirs
The reservoir of influenza type B and C is typically only humans. Influenza type A, however, can infect both humans and animals, such as wild birds, poultry, pigs, horses, mink, and ferrets. The primary reservoir for influenza D is cattle. However, it may also infect pigs.
In recent years, highly pathogenic avian influenza (HPAI), specifically influenza A(H5N1), has affected large numbers of wild and domestic birds in Colorado, the United States, and worldwide. Dairy cattle have also been affected. These avian influenza A viruses usually do not infect people, but there have been sporadic cases of human infection, primarily among people who are exposed to sick animals through their work. Public health should monitor people who had contact with or exposure to HPAI-infected birds or other animals for signs of infection.
Modes of transmission
Influenza is transmitted through droplets when someone who is infected coughs, sneezes, or talks. These pathogen-containing droplets can land in the mouth or nose of someone within about six feet. Although less likely, influenza can also be spread if a person touches a contaminated surface or object and then touches their eyes, nose, or mouth.
Incubation period
The incubation period of influenza ranges from 1 to 4 days (average of two days).
Infectious period
The virus can be detected in people one day prior to symptom onset and up to 5 to 7 days after becoming ill. People infected with influenza are most contagious in the first 3-4 days after their symptom onset. However, younger children and people with weakened immune systems can be contagious for more than seven days.
Epidemiology
Seasonal influenza epidemics most often occur in the fall and winter months and typically peaks anywhere from December to February in the United States. The CDC estimates that influenza has resulted in 9.3 million to 41 million illnesses, 120,000 to 710,000 hospitalizations, and 6,300 to 52,000 deaths annually in the United States between 2010 and 2024. During the COVID-19 pandemic, shifts in the circulation patterns of influenza viruses occurred which led to periods of decreased and interseasonal activity.
- Influenza-associated hospitalization: hospitalized patient who test positive for influenza within 14 days prior to admission or at any time during an admission
- Influenza-associated pediatric death: death of a child younger than 18 years with a compatible illness confirmed to be influenza by a test taken either before or after death
Laboratory criteria for diagnosis
Confirmed
Influenza A and/or B identified by one of the following laboratory tests: rapid molecular assay, reverse transcriptase polymerase chain reaction or other nucleic acid amplification, viral culture, or rapid antigen (rapid influenza diagnostic tests and immunofluorescence assays). Refer to the CDC website for additional details about influenza testing methods.
Case classification
Confirmed
Meets hospitalization/death and laboratory criteria
Outbreak definitions
Nursing/intermediate care/assisted living/group home and correctional facilities
- Suspect: at least one confirmed case of influenza among residents/incarcerated people and at least one other resident/incarcerated person with undiagnosed respiratory illness with symptom onset dates within a one-week period
- Confirmed: at least two confirmed cases of influenza among residents/incarcerated people with test dates within a one-week period
There are no defined outbreak thresholds for other settings. Consult with your local public health agency or CDPHE. Refer to the following guidance documents for information on responding to viral respiratory disease outbreaks in specific settings.
Situations in which the Colorado State Public Health Laboratory (“State Lab”) may provide influenza testing services includes but is not limited to:
- Influenza A subtyping for hospitalized patients at hospitals that cannot or will not be subtyped by the clinical laboratory
- Influenza A subtyping on any clinical specimen where the clinical laboratory result is unsubtypeable
- Influenza testing on any clinical specimen where the clinical laboratory result is non-standard (e.g., detection of both influenza A and B, detection of two influenza A subtypes)
- Influenza testing on specimens submitted as part of the sentinel laboratory surveillance program
- Influenza testing for select outbreaks or people exposed to HPAI who are symptomatic on a case-by-case basis in consultation with the CDPHE Viral Respiratory Diseases Program
Guidance for submitting samples, as well as other helpful resources, can be found on the State Lab’s webpage.
- Additional questions can be directed to the Lab Coordinators by email (cdphe_labcoordinators@state.co.us; preferred method) or phone 303-692-3069.
Treatment
Influenza can cause a spectrum of disease in infected individuals. CDC recommends starting antiviral treatment as soon as possible when flu is suspected or diagnosed in someone who is hospitalized; has severe, progressive, or complicated illness (such as pneumonia or worsening of an underlying condition); or is at higher risk of influenza complications (including all adults 65 years and older, all children younger than 5 years, and those who are pregnant or have certain underlying conditions). Starting treatment in these priority groups should not wait for influenza testing results. Antiviral treatment has the greatest clinical benefit when started within 48 hours of symptom onset. However, starting treatment later can still be beneficial for people in the priority groups. There are four antiviral medications approved and recommended for treatment of influenza in the United States: oseltamivir (Tamiflu®), zanamivir (Relenza®), peramivir (Rapivab®), and baloxavir marboxil (Xofluza®). Oseltamivir is preferred for treatment of influenza during pregnancy and is specifically recommended as soon as possible for hospitalized patients and non-hospitalized patients with complications or progressive disease. For more information, see CDC’s webpage about treating flu with antiviral drugs.
Prophylaxis
Routine seasonal influenza vaccination is recommended for all people aged 6 months and older who do not have contraindications. CDC recommends vaccination before the end of October, and that vaccination should continue throughout the season as long as influenza viruses are circulating and vaccine is available. It is important, and often required, that health care professionals who work in health care settings (e.g., hospitals, long-term care facilities) receive influenza vaccination annually. For information on influenza and other vaccines in Colorado, refer to CDPHE’s immunization resource webpage.
In certain situations, antiviral medication can be administered as post-exposure chemoprophylaxis. For example, in response to a confirmed influenza outbreak in a long-term or other residential care facility, antiviral chemoprophylaxis is recommended for all medically eligible, asymptomatic residents (regardless of whether they received an influenza vaccine). Chemoprophylaxis can be considered for staff according to IDSA Influenza Clinical Practice Guidelines. For additional information, see CDPHE’s guidance for nursing and immediate care facilities and assisted living and other group homes.
Education
Certain health behaviors can help prevent becoming ill with or spreading the influenza virus. Precautions to help reduce the spread of influenza include:
- Washing hands thoroughly and often
- Avoiding touching eyes, nose, and mouth
- Avoiding contact with people who are ill
- Staying home when ill*
- Covering coughs and sneezes
- Cleaning/disinfecting surfaces
*Someone who is confirmed or suspected to have influenza should stay home for at least 24 hours after their fever has resolved (without the use of fever-reducing medications) and respiratory symptoms are improving, except to get medical care or other necessities.
Managing special situations
Centers for Disease Control and Prevention (CDC). Influenza (Flu). Atlanta, Georgia. Department of Health and Human Services. Available from URL: https://www.cdc.gov/flu. Accessed July 25, 2025.
CDC. Flu burden. Atlanta, Georgia. Department of Health and Human Services. Available from URL: https://www.cdc.gov/flu-burden/php/about/index.html. Accessed Aug. 5, 2025.
CDC. Morbidity and Mortality Weekly Report (MMWR). Changes in Influenza and Other Respiratory Virus Activity During the COVID-19 Pandemic — United States, 2020–2021. Atlanta, Georgia. Department of Health and Human Services. July 23, 2021. Available from URL: https://www.cdc.gov/mmwr/volumes/70/wr/mm7029a1.htm
CDC. U.S. Influenza Surveillance: Purpose and Methods. Atlanta, Georgia. Department of Health and Human Services. Available from URL: https://www.cdc.gov/fluview/overview. Accessed July 25, 2025.
CDC. National Notifiable Disease Surveillance System (NNDSS). Influenza-associated Hospitalizations 2012 Case Definition. Atlanta, Georgia. Department of Health and Human Services. Available from URL: https://ndc.services.cdc.gov/case-definitions/influenza-associated-hospitalizations-2012/.
CDPHE Communicable Disease Branch
- Phone: 303-692-2700 or 800-866-2759
- Fax: 303-782-0338
- After hours: 303-370-9395
CDPHE Lab Coordinators:
- Email (preferred): cdphe_labcoordinators@state.co.us
- Phone: 303-692-3480