Pulmonary

IDSA Clinical Practice Guidelines: Seasonal Influenza

Summary:

Seasonal influenza is a respiratory illness caused by the Influenza A and B viruses. Virus outbreaks typically occur in the winter season, and symptoms can range from mild to life threatening. While most patients recover from uncomplicated influenza, the illness can progress to severe disease and death. Young children, older adults, pregnant and postpartum patients, and those with certain medical conditions such as immunosuppression, are at highest risk of complications. While influenza rates declined markedly during the COVID-19 pandemic, the virus returned aggressively in 2022 with unprecedented rates of infection compared to recent years. The cornerstone of influenza treatment remains prevention with yearly vaccination. The IDSA last released a clinical practice guideline in 2018 to assist with diagnosis, treatment, chemoprophylaxis and outbreak management.

Diagnosis:

Signs and Symptoms

Commonly: Fever | Myalgia | Non-productive cough
Also typical: Sore Throat | Rhinorrhea | Headache | Nausea | Weakness
Complications: Pneumonia | Respiratory failure | ARDS | Multiorgan failure | Sepsis | Liver inflammation| Rhabdomyolysis | Myocarditis | Encephalitis | AKI
Laboratory, physical exam, and imaging findings are not specific for the flu and generally do not clarify diagnosis in the absence of flu testing

Outpatient Testing

Clinicians should preform outpatient testing for the flu during an influenza outbreak in patients who:
- Are high risk (e.g. older age, immunocompromise, pregnant) and present with acute respiratory illness or flu-like symptoms if testing will alter clinical management
- Develop acute respiratory symptoms, with or without fever, in addition to exacerbation of chronic medical conditions or known complication of the flu (e.g. pneumonia) and testing will alter clinical management
- Are not high risk but present with flu-like illness, pneumonia or respiratory illness and are likely to be discharged or reduce the use of antibiotics in the setting of a positive flu test, or otherwise alter clinical management
Clinicians can consider outpatient testing for the flu in the absence of high local flu activity if a patient presents with acute respiratory symptoms, prioritizing those with immunocompromise or high-risk conditions

Inpatient Testing

Clinicians should preform inpatient testing for the flu during an influenza outbreak in patients who:
- Are being admitted with acute respiratory illness, with or without fever
- Are being admitted for acute worsening of chronic cardiopulmonary disease (e.g. asthma, CHF, COPD)
- Are being admitted with respiratory symptoms, with or without fever, in the setting of immunocompromise or high-risk comorbidities
- Develop respiratory symptoms while hospitalized in the absence of a clear alternative diagnosis
Clinicians should test for the flu during low local influenza activity if the patient is being admitted for acute respiratory illness and has: Known exposure to the flu | Exposure to a respiratory illness outbreak of uncertain cause | Traveled from an area with high flu activity
Clinicians can consider testing for influenza in patients who present with acute febrile respiratory illness and are at high risk of complications, if testing will alter management

Modes of Testing

Clinicians should obtain upper respiratory specimens for testing as soon as symptoms develop, and preferably within 4 days to improve yield and accuracy
Nasopharyngeal specimens are preferred
- Endotracheal aspirate or BAL specimens can be used for inpatients on mechanical ventilation
Outpatient testing:
- Rapid molecular assays (e.g. nucleic acid amplification tests) are preferred over rapid diagnostic tests
Inpatient testing:
- Reverse-transcription polymerase chain reaction (RT-PCR) or other molecular assays are preferred
- RT-PCR targeting multiple respiratory pathogens should be used in immunocompromised patients or in immunocompetent patient’s if it will influence care
Consider resistance testing for:
- Patients who develop influenza infection while on chemoprophylaxis
- Immunosuppressed patients or patients with severe disease with evidence of ongoing disease after 7 to 10 days
- Patients who received subtherapeutic doses of NAI therapy

Treatment:

Start antiviral therapy as soon as possible in all patients with positive flu testing who meet the following criteria:
- Inpatients hospitalized for influenza infection, regardless of duration
- Outpatients with severe or progressive illness, regardless of duration
- Outpatients at high risk for complications
- Children < 2 years old and adults ≥ 65 years old
- Pregnant patients and those within 2 weeks postpartum
Consider antiviral therapy in all patients not at risk for complications who the meet the following criteria:
- Outpatients who present within 48 hours of symptom onset
- Outpatients with symptoms who are household contacts of those at high risk of complications
- Outpatient symptomatic healthcare workers who care for patients
  who are at high risk of developing complications if infected with influenza
Antiviral therapy is with: Oral oseltamivir | Inhaled Zanamivir | Single dose of intravenous Peramivir
Oral and inhaled therapies should be given for 5 days, but course may be extended for immunosuppressed and hospitalized patients
Further work up for alternative causes and antibiotics for bacterial coinfection should be added for patients who: Present with severe disease | Improve but then deteriorate | Fail to improve after 3 to 5 days of antiviral therapy

Chemoprophylaxis:

Oral and inhaled NAIs are the preferred drugs for chemoprophylaxis
Post exposure prophylaxis should be given with 48 hours of exposure and last for 7 days since last contact with known exposure
If a patient on chemoprophylaxis tests positive for the flu they should be switched to antiviral treatment dosing

Preexposure Chemoprophylaxis

Preexposure chemoprophylaxis for the length of flu season may be considered for:
- Patients at high risk of flu complications who are unable to be vaccinated (e.g., the severely immunosuppressed) or unlikely to respond to vaccines (e.g., recent stem cell transplant patients)
Short term preexposure chemoprophylaxis and administration of inactivated flu vaccine may be considered for:
- Patients at high risk for flu complications in whom the vaccine is expected to be effective during times of high flu activity

Postexposure Chemoprophylaxis

Consider giving postexposure chemoprophylaxis to Asymptomatic high-risk patients who are unable to be adequately vaccinated with a household exposure
Consider giving postexposure chemoprophylaxis and the inactivated influenza vaccine to unvaccinated patients with household members at high risk for complications

Institutional Outbreak Control:

Outbreak control measures include: Chemoprophylaxis for residents/patients for 14 days and at least 7 days after last symptom onset of last identified case| Surveillance testing | Testing for all patients who develop respiratory symptoms | Empiric treatment dose antiviral therapy for patients who develop symptoms prior to test results