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Detection And Control Of Influenza Outbreaks In Acute Care Facilities

1. What is the impact and risk?
When to conduct surveillance
2. What are the symptoms?
Why is lab testing important?
3. What are the complications?
When should testing be done?
4. How is it transmitted?
How to confirm a diagnosis
5. Incubation period
Control measures
6. How to prevent outbreaks
How to prepare for outbreaks
7. Why conduct surveillance?   Lab Diagnostic Table
How to conduct surveillance  

1. What is the impact of influenza and who is at risk?

Annual influenza epidemics cause infection in 10-20 % of the population and result in an average of >110,000 hospitalizations and 20,000 deaths in the United States. Persons most susceptible to complications or death from influenza are:

  • persons >50 years of age
  • children <2 years of age
  • persons of any age with chronic medical conditions


2. What are the symptoms of influenza infection?

Influenza is a respiratory illness characterized by the abrupt onset of fever, chills, headache, body aches, and fatigue with accompanying cough, sore throat, and nasal congestion. While the fever, body aches, and headache may typically last for three to five days, the cough and fatigue may last for more than two weeks. Some persons may not have typical influenza symptoms, but present with worsening of chronic medical conditions. Some children primarily may have fever, with nausea, vomiting or abdominal pain, and infants may have symptoms similar to a severe systemic bacterial infection.

3. What are the complications from influenza infection?

The most common complications are secondary bacterial pneumonia and worsening of chronic medical conditions. Influenza virus also can cause primary pneumonia, but is much less common. Rarely, children taking aspirin can develop Reye's syndrome if they get sick with influenza.

4. How is influenza transmitted?

Influenza is easily transmitted from person-to-person. The virus is spread primarily by the coughing and sneezing of infected persons or sometimes, by direct contact, either with infected persons or a contaminated surface. Once influenza is introduced into a facility by infected healthcare personnel, patients, or visitors, it can quickly spread and cause illness in other hospitalized patients and healthcare personnel, especially in those who are unvaccinated. During a hospital ward or nursing home outbreak, as many as 70% of staff and patients may become infected.

5. What is the incubation period for influenza and how long is a person contagious?

Infected persons start to develop symptoms 1-4 days after they are exposed. They may be able to spread influenza to other people from the day before getting symptoms through 5-7 days after symptoms start. Children may be contagious for 7 or more days.

6. How can influenza outbreaks in health care facilities be prevented?

The most important means to prevent influenza illness from spreading in a health care facility is influenza vaccination of both patients and healthcare personnel. The Advisory Committee on Immunization Practices recommends annual vaccination of all healthcare personnel. However, in part due to low vaccination rates, less than 100% efficacy, and because influenza-infected patients will be admitted from the community, outbreaks of influenza can occur. When influenza is introduced into a health care facility, prompt recognition of influenza infection and initiation of infection control measures can limit the spread of disease.

7. Why should acute care facilities conduct surveillance for influenza and influenza-like illness?

An active surveillance program for influenza and influenza-like illness can help acute care facilities identify outbreaks of influenza early in their course and prevent influenza from spreading to patients and healthcare personnel, thereby decreasing influenza-related complications among patients and reducing work absenteeism. When the onset of influenza season in the community is identified, facility leaders should initiate measures to increase awareness and intensify efforts to diagnose and prevent influenza illness in both patients and healthcare personnel.

8. How should surveillance be conducted?

Surveillance can be conducted in a number of settings including: 1) Inpatient surveillance

  • Document incidence of reported influenza and influenza-like illness
  • Develop case definitions for healthcare facility-acquired and community-acquired influenza
  • Consider patients who develop influenza-like illness >72 hours after facility admission as potential cases of health care facility-acquired influenza-like illness.
  • Initiate influenza testing and droplet precautions when healthcare facility-acquired influenza is detected during surveillance, particularly when:
    • one or more patients are identified with healthcare facility-acquired laboratory confirmed influenza
    • a cluster of (e.g., >3) patients with healthcare facility-acquired influenza-like illness are identified on the same floor or ward during a short (e.g., 48-72 hour) period.
  • Consider daily monitoring for influenza-like illness in selected settings, especially on wards with particularly vulnerable patients, such as intensive care units, oncology units, and other "sentinel" floors.
  • Evaluate whether infection control measures are properly instituted for influenza-positive patients, and investigate whether the infection was acquired in the community or while hospitalized.

2) Employee surveillance

  • Consider requiring that healthcare personnel with influenza-like illness go to employee health services for influenza testing.
  • Exclude healthcare personnel who test influenza-positive or have influenza-like illness from care of patients at high risk for influenza complications.

3) Emergency Department surveillance

  • Consider influenza testing of patients being admitted from the emergency department who have influenza-like illness with no other identified pathogen.
  • Facilitate the timely initiation of droplet precautions, through early diagnosis among patients being admitted, to lessen the chance of influenza spreading to personnel or other patients.

4) Laboratory surveillance

  • Infection control personnel should be in regular contact with laboratory personnel regarding influenza-positive specimens.
  • Update clinical personnel regularly about the availability and use of diagnostic tests.


9. When should surveillance be conducted?

Acute care facilities should conduct surveillance for health care facility-acquired influenza or influenza-like illness, particularly during the influenza season from October through April. However, sporadic cases of influenza can occur at any time of the year.

10. Why is laboratory testing for influenza important?

Influenza is very difficult to diagnose on the basis of clinical symptoms alone. Other pathogens that can cause similar symptoms include, but are not limited to, Mycoplasma pneumoniae, adenovirus, respiratory syncytial virus, rhinovirus, parainfluenza viruses, and Legionella spp.

11. When should influenza testing be done?

Based on facility surveillance, infection control personnel should develop threshold levels of influenza or influenza-like illness at which influenza testing and outbreak control measures should be initiated. Physicians may have a lower threshold for testing individuals at high risk for influenza-related complications. Droplet precautions should be initiated pending laboratory confirmation of influenza.

12. What laboratory tests can be used to confirm the diagnosis of influenza?

Appropriate samples to collect can include a nasopharyngeal or throat swab, from adults or nasal wash, or nasal aspirates, depending on which rapid test is used. Samples should be collected within the first 4 days of illness. Rapid influenza tests provide results within 24 hours; viral culture provides results in 3-10 days. Most of the rapid tests are approximately >70% sensitive for detecting influenza and approximately >90% specific. Because as many as 30% of samples that would be positive for influenza by viral culture may give a negative rapid test result, negative rapid tests should be followed by viral culture in a sub-sample of the swabs collected. Viral culture can also identify other causes of influenza-like illness when influenza is not the cause.

Serum samples can be tested for influenza antibody to diagnose acute infections. Two samples should be collected per person: one sample within the first week of illness and a second sample 2-4 weeks later. If antibody levels increase from the first to the second sample, influenza infection likely occurred. Because of the length of time needed for a diagnosis of influenza by serologic testing, other diagnostic testing should be used for rapid detection of possible outbreaks.

See table on influenza laboratory diagnostic procedures at the end of the document.

13. What control measures should be used for influenza or influenza-like illness?

a) Cohort those with influenza or influenza-like illness together on a ward designated to accept patients with suspected or confirmed influenza.

b) Initiate droplet precautions for persons with influenza-like illness or confirmed influenza infection, including wearing masks when within 3 feet of the patient, wearing gowns if clothing is likely to be soiled by body fluids, and washing hands before and after patient contact.

c) Offer influenza vaccine to patients and healthcare personnel who have not been vaccinated. Healthcare personnel vaccination is critical in preventing influenza transmission.

d) Consider offering influenza antiviral medications for treatment of ill patients and healthcare personnel and for prophylaxis of exposed patients, unvaccinated personnel, and those vaccinated <2 weeks before exposure.

e) Monitor personnel for influenza-like illness and restrict ill personnel from patient care.

f) Restrict visitors with influenza-like illness.

g) Continue to monitor for healthcare facility-acquired influenza and for patients being admitted to the facility who have influenza infection.

14. What can acute care facilities do to prepare for possible influenza outbreaks?

a) Make sure that all personnel receive influenza vaccination by:

  • improving access to vaccine (e.g., using a mobile cart to vaccinate healthcare personnel in their work areas, at conferences, in lunchrooms, or in other meeting areas).
  • addressing reminders from the employee health department to all healthcare personnel and their supervisors. The reminders should include a list of places and times that vaccination will be offered.
  • annually providing reports of vaccination levels by employee unit. Information should be disseminated to personnel and their supervisors and may be used to foster a spirit of competition. Set target vaccination goals and publicize running vaccination totals throughout the facility during vaccination periods. Nominal awards may be beneficial in improving coverage and should be considered. Recognition should be given to employee units with high coverage levels.
  • educating personnel annually about the risks of influenza to their patients, themselves, and their families, and about the benefits of vaccination. Personnel should receive data about the impact of influenza, if known, in their facility. Ideally, vaccination should be offered at the end of such education sessions.

b) Offer vaccine to unvaccinated patients before they are discharged.

c) Have a written policy concerning influenza outbreak management and ensure that key healthcare personnel, especially nurses, are aware of it.

d) Disseminate information about influenza testing and use of influenza antiviral medication to physicians.

e) Institute surveillance for influenza-like illness among healthcare personnel and patients.

Laboratory diagnostic procedures for influenza1
Procedure Influenza
Time for
Viral culture A and B NP swab2
throat swab
nasal wash
bronchial wash
nasal aspirate
Immunofluorescence A and B NP swab2
nasal wash
bronchial wash
nasal aspirate
Influenza A (EIA)
Enzyme Immuno Assay
A and B NP swab2
throat swab
nasal wash
brochial wash
Directigen A
A NP swab2
throat swab
nasal wash
nasal aspirate
FLU OIA (Biostar) A and B4 NP swab2
throat swab
nasal aspirate
Quick Vue (Quidel) A and B4 NP swab2
nasal wash
nasal aspirate
Zstat Flu (ZymeTx) A and B4 throat swab <30
RT-PCR A and B NP swab2
throat swab
nasal wash
bronchial wash
nasal aspirate
Serology A and B paired acute and

1 List may not include all test kits approved by the U.S. Food and Drug Administration
2 NP = nasopharyngeal
3 Shell vial culture, if available, may reduce time for results to 2 days
4 Does not distinguish between influenza A and B types
5 A fourfold or greater rise in antibody titer from the acute- (collected within the 1st week of illness) to the convalescent-phase (collected 2-4 weeks after the acute sample) samples is indicative of recent infection.

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Editor: Susan K. Boyer, RN
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