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

As of September 2001, 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 >65 years of age
• children <2 years of age
• persons of any age with chronic medical conditions

Update (December 2003): For updated incidence and mortality data click here.

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2. What are the symptoms of influenza infection?

Influenza is a respiratory illness characterized by the abrupt onset of fever, chills, headache, myalgias, i.e. body aches with accompanying fatigue 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 exhibit exacerbation 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.
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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.
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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 an outbreak in a hospital ward or nursing home, as many as 70% of staff and patients may become infected.
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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.
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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.
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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.
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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 (e.g., droplet precautions) are properly instituted for influenza-positive patients, and investigate whether the infection was acquired in the community or while hospitalized.

2) 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.

3) Employee surveillance

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

4) Laboratory surveillance

• Establish regular contact between infection control and laboratory personnel regarding influenza-positive specimens.
• Update clinical personnel regularly about the availability and use of diagnostic tests.

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Acute care facilities should conduct surveillance for acute care facility-acquired influenza or influenza-like illness, particularly during the influenza season (from October through April in North America). However, sporadic cases of influenza can occur at any time of the year.
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Influenza is very difficult to differentiate from other pathogens 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. Many pathogens, including influenza, RSV, and parainfluenza, cause outbreaks in a seasonal pattern.
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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.
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Update (December 2003): For new flu tests click here

Appropriate patient samples to collect for laboratory testing 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 >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.

Update (December 2003): For updated table on influenza laboratory diagnostic procedures click here
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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 practicing hand hygiene before and after patient contact.

c) Offer influenza vaccine to patients and healthcare personnel who have not been vaccinated.

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.

Update (December 2003): For updated clinical information click here

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.
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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).
• 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.
• 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.
• 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 to physicians about influenza testing and use of influenza antiviral medication.

e) Institute surveillance for influenza-like illness among healthcare personnel and patients.
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Influenza Exposure Flow Chart

This page last reviewed December 31, 2003