H1N1 influenza: evaluation and management

1. Clinical assessment

• Abrupt fever or chills, headache, prominent myalgia, profound fatigue
• Dry cough, sore throat, rhinorrhea
• Sometimes substernal burning/pain and GI symptoms, especially in children

• Vitals including respiratory rate, oxygen saturation, mental status, hydration
• Respiratory distress, cyanosis, focal chest signs, wheeze, or sepsis features
• Look for pneumonia:
  • Primary viral pneumonia: rapidly worsening dyspnea, hypoxemia, diffuse bilateral interstitial infiltrates
  • Secondary bacterial pneumonia: recurrent fever and productive cough after initial improvement, often 1-2 weeks later. S. pneumoniae and S. aureus, including MRSA, are important pathogens. [Fishman's Pulmonary Diseases and Disorders, p. 2221]

• Age under 2 years or 65 years and over
• Pregnancy or up to 2 weeks postpartum
• Chronic pulmonary, cardiac, renal, hepatic, neurologic, metabolic disease, or diabetes
• Immunocompromise
• BMI 40 kg/m² or above
• Nursing-home or long-term-care residents

2. Testing

• Do not delay treatment for test results in hospitalized, severely ill, progressive, or high-risk patients with suspected influenza.
• Use a rapid molecular assay or RT-PCR when confirmation will influence care, infection-control decisions, or management of complications.
• A rapid antigen test can be used, but a negative result does not exclude influenza, particularly when influenza is circulating. Confirm with molecular testing if suspicion remains high. The older rapid tests have only moderate sensitivity. [Textbook of Family Medicine, p. 274]
• In severe disease, also consider:
  • CBC, renal function, electrolytes, liver tests as clinically indicated
  • Chest radiograph for dyspnea, hypoxemia, abnormal lung examination, or suspected pneumonia
  • Blood/sputum cultures if severe pneumonia or sepsis is suspected
  • SARS-CoV-2 testing when relevant, since presentations overlap

3. Antiviral treatment

1. Hospitalized patients
2. Severe, complicated, or progressive illness, including pneumonia or exacerbation of chronic disease
3. Any patient at high risk of influenza complications

• Consider an antiviral if treatment can begin within 48 hours of onset, particularly if symptoms are significant or household transmission risk is important.
• Supportive care alone is reasonable for mild illness presenting later than 48 hours without risk factors or progression.

• Oseltamivir: adults, 75 mg orally twice daily for 5 days.
  • Adjust dose for renal impairment.
  • Common adverse effects: nausea and vomiting. Taking it with food may help.
• Hospitalized or severe influenza: oral or enteric oseltamivir is preferred; longer treatment may be considered for prolonged severe illness or ongoing viral replication, guided by the treating clinician.
• Alternatives for uncomplicated outpatient influenza, where appropriate:
  • Baloxavir: single oral dose, generally within 48 hours; avoid in pregnancy and use cautiously in immunocompromised patients.
  • Zanamivir inhaled: avoid in asthma, COPD, or underlying airway disease due to bronchospasm risk.
  • Peramivir IV: an option when oral treatment is unsuitable.

4. Supportive care and infection prevention

• Fluids, rest, acetaminophen or ibuprofen for fever/pain when appropriate
• Avoid aspirin in children and adolescents because of Reye syndrome risk
• Stay home until fever-free for at least 24 hours without fever-reducing drugs, and follow local public-health or workplace rules
• Masking, hand hygiene, ventilation, and avoiding close contact with high-risk people reduce spread
• Annual influenza vaccination remains the key preventive measure. Current seasonal vaccines include an H1N1 component.

5. Antibiotics and corticosteroids

• Antibiotics are not indicated for uncomplicated H1N1/influenza.
• Give antibiotics only if there is suspected or confirmed bacterial pneumonia, sepsis, otitis media, or another bacterial complication.
• Do not give systemic corticosteroids solely for influenza pneumonia. Use them only for another clear indication, such as an asthma/COPD exacerbation or refractory septic shock.

6. Refer urgently or admit if any of the following

• Trouble breathing, respiratory rate elevation, oxygen saturation low or falling
• Chest pain, cyanosis, confusion, persistent drowsiness, seizures
• Inability to drink, severe dehydration, persistent vomiting
• Signs of sepsis or shock
• Rapid clinical deterioration or recurrence of fever with productive cough after initial recovery
• Any concerning symptoms in pregnancy, infants, frail older adults, or immunocompromised people

H1N1 influenza: evaluation and management

Evaluation

• Sudden fever/chills, headache, myalgia, marked malaise
• Dry cough, sore throat, rhinorrhea
• Children may have vomiting or diarrhea
• In uncomplicated infection, chest examination may be normal. [Fishman's Pulmonary Diseases and Disorders, p. 2221]

• Vital signs, oxygen saturation, respiratory rate, hydration and mental state
• Examine for pneumonia, respiratory distress, wheeze, dehydration, or sepsis.
• Obtain chest radiograph if hypoxemia, dyspnea, focal chest signs, or suspected pneumonia.
• In severe illness consider CBC, renal/liver function, electrolytes, blood culture and sputum culture.

• Primary viral pneumonia/ARDS: rapid deterioration, dyspnea, cyanosis, hypoxemia, bilateral interstitial infiltrates.
• Secondary bacterial pneumonia: recurrent fever and productive cough after initial improvement. Common organisms are Streptococcus pneumoniae and Staphylococcus aureus, including MRSA. [Fishman's Pulmonary Diseases and Disorders, p. 2221]

Diagnostic testing

• Use RT-PCR or a rapid molecular influenza assay when confirmation will affect management, admission, infection precautions, or outbreak control.
• Rapid antigen tests can be useful but have limited sensitivity. A negative result does not rule out influenza when clinical suspicion is high.
• Test for COVID-19 as appropriate because presentations overlap.
• Do not wait for test results before starting antiviral therapy in severe, hospitalized, progressive, or high-risk cases. Current CDC clinical guidance supports empiric treatment in these groups.

Who needs antiviral treatment?

1. Hospitalized patients
2. Severe, progressive, or complicated disease, including pneumonia or exacerbation of asthma/COPD/heart failure
3. High-risk patients, including:
   • Age under 2 years or 65 years and over
   • Pregnancy and up to 2 weeks postpartum
   • Chronic heart, lung, kidney, liver, neurologic, metabolic disease or diabetes
   • Immunosuppression
   • BMI 40 kg/m² or higher
   • Residents of long-term care facilities

Antiviral management

• Oseltamivir 75 mg orally twice daily for 5 days in adults with uncomplicated infection.
• Dose adjustment is required in renal impairment.
• Give oral/enteric oseltamivir promptly to hospitalized or severe patients. Clinicians may extend therapy in prolonged or critical illness.

• Baloxavir: single oral dose when started within 48 hours. Avoid in pregnancy; do not use routinely in severe hospitalized influenza or immunocompromised patients.
• Zanamivir inhaled: avoid in asthma or COPD because of bronchospasm risk.
• Peramivir IV: option if oral treatment is not feasible.

Supportive care

• Oral fluids, rest, antipyretics/analgesia such as acetaminophen or ibuprofen if appropriate
• Avoid aspirin in children/adolescents because of Reye syndrome risk
• Give oxygen for hypoxemia and manage respiratory failure according to severity
• Use antibiotics only for suspected bacterial coinfection or secondary bacterial pneumonia
• Do not use systemic corticosteroids solely for influenza pneumonia unless there is another indication, such as asthma/COPD exacerbation or refractory septic shock.

Admission or urgent referral

• Breathlessness, cyanosis, hypoxemia, chest pain
• Confusion, reduced consciousness, seizures
• Persistent vomiting or inability to maintain fluids
• Hypotension, sepsis, shock
• Rapid clinical decline or recurrence of fever after initial improvement
• Any significant illness in pregnancy, infancy, old age, or immunocompromise

Prevention and infection control

• Annual influenza vaccination
• Stay home while acutely unwell and until fever has resolved for at least 24 hours without antipyretics, subject to local policy
• Masking around others, hand hygiene, respiratory etiquette, and good ventilation
• Consider antiviral post-exposure prophylaxis selectively for high-risk exposed individuals, based on local guidance.

• Salbutamol/albuterol reliever inhaler or nebulizer: may help only if there is wheezing, bronchospasm, or an asthma/COPD flare triggered by influenza. It eases airway tightening but does not treat the H1N1 virus or viral pneumonia. Use it as prescribed in your asthma/COPD action plan.
• Preventer steroid inhalers such as budesonide or beclomethasone: continue your usual prescribed controller inhaler. Do not start one solely to treat flu without clinician advice.
• Systemic steroids such as prednisolone should not be routinely used for influenza, but may be needed for a genuine asthma or COPD exacerbation under medical supervision.
• Zanamivir is a different inhaled medication: it is an antiviral for influenza, not a bronchodilator. Avoid it in asthma or COPD because it can cause wheezing/bronchospasm. CDC guidance for asthma