What is the first line of treatment for a patient presenting with flu symptoms, particularly for those with underlying health conditions or at high risk for complications?

First-Line Treatment for Influenza

For patients presenting with flu symptoms, particularly those with underlying health conditions or at high risk for complications, initiate oseltamivir 75 mg orally twice daily for 5 days immediately upon clinical suspicion, without waiting for laboratory confirmation, and ideally within 24 hours of symptom onset. 1, 2

Immediate Antiviral Treatment Decision

Start oseltamivir immediately for all high-risk patients presenting within 48 hours of symptom onset, as this reduces illness duration by approximately 24 hours and may prevent serious complications including hospitalization and death 1, 2. The benefit is greatest when treatment begins within 24 hours, but effectiveness extends progressively through 36 hours 3, 4.

High-Risk Populations Requiring Immediate Treatment:

• Children <2 years and adults ≥65 years 2
• Pregnant women and those within 2 weeks postpartum 2
• Immunocompromised patients, including those on immunomodulator therapy 3, 2
• Patients with chronic medical conditions (COPD, heart disease, diabetes, asthma, neurologic disorders) 3, 2

Critical caveat: Do not delay treatment while awaiting diagnostic test results, as rapid antigen tests have low sensitivity and negative results should not be used to rule out influenza 3.

Treatment Beyond 48 Hours

For hospitalized or severely ill patients, initiate oseltamivir even if >48 hours from symptom onset, as observational studies demonstrate reduced morbidity and mortality with delayed treatment in severe cases 3, 1, 5. This recommendation applies particularly to immunocompromised patients 3, 2.

Dosing Regimens

Adults and Children >40 kg:

• Treatment: Oseltamivir 75 mg orally twice daily for 5 days 3, 1
• Chemoprophylaxis: 75 mg once daily for 10 days 3

Pediatric Weight-Based Dosing (Children >12 months):

• ≤15 kg: 30 mg twice daily 3
• >15-23 kg: 45 mg twice daily 3
• >23-40 kg: 60 mg twice daily 3
• >40 kg: 75 mg twice daily 3

Infants 3-12 months:

• 3 mg/kg/dose twice daily (though not FDA-approved, this dosing can be followed based on 2009 H1N1 pandemic recommendations) 3

Important tolerability note: Nausea and vomiting occur in approximately 15% of patients but are typically mild, transient, and reduced when taken with food 5, 6, 7.

Alternative Antiviral Options

Zanamivir (inhaled) 10 mg twice daily for 5 days is an alternative for patients who cannot tolerate oseltamivir or when oseltamivir resistance is suspected 3, 5. However, zanamivir should be used cautiously in patients with underlying respiratory disease 8.

Supportive Care for All Patients

Regardless of antiviral eligibility, provide:

• Acetaminophen or ibuprofen for fever, myalgias, and headache 1
• Adequate hydration and rest 1
• Absolute contraindication: Aspirin in children <16 years due to Reye's syndrome risk 1

When to Add Antibiotics

Antibiotics are NOT routinely indicated for uncomplicated influenza 1, 5. However, consider empiric antibiotic therapy if:

• Worsening symptoms after initial improvement (suggests bacterial superinfection) 1, 5
• Recrudescent fever or increasing breathlessness 5
• No improvement after 3-5 days of antiviral therapy 2
• Features of pneumonia on examination or chest X-ray 1

First-Line Antibiotic Choices:

• Co-amoxiclav (amoxicillin-clavulanate) 625 mg three times daily for 7 days OR
• Doxycycline 200 mg loading dose, then 100 mg once daily for 7 days 1, 5

These regimens cover the most common bacterial superinfections: Staphylococcus aureus (including MRSA), Streptococcus pneumoniae, and Haemophilus influenzae 5.

Severity Assessment and Hospital Referral

Use CURB-65 score for pneumonia stratification, with urgent hospital referral required for:

• CURB-65 score ≥3 1
• Bilateral lung infiltrates on chest X-ray 1
• Oxygen saturation <90% 1
• Inability to maintain oral intake 5
• Altered mental status 5

Chemoprophylaxis Indications

Consider oseltamivir 75 mg once daily for 10 days for:

• Unvaccinated household contacts of confirmed cases (start within 48 hours of exposure) 3, 7
• High-risk patients during the 2 weeks after influenza vaccination 3
• Immunocompromised patients as adjunct to vaccination 3
• Outbreak control in institutional settings 3

Important timing: Both oseltamivir and zanamivir decrease risk of symptomatic infection when given in the early phase after close contact 3.

Critical Pitfalls to Avoid

1. Do not wait for laboratory confirmation before starting treatment in high-risk patients 2
2. Do not rely on rapid antigen tests to rule out influenza due to low sensitivity 3
3. Do not withhold antivirals beyond 48 hours in severely ill or hospitalized patients 3, 5
4. Do not routinely prescribe antibiotics without evidence of bacterial superinfection 1
5. Avoid live attenuated influenza vaccine (LAIV) within 2 weeks before or 48 hours after oseltamivir due to potential interference 6

Prevention Remains Paramount

Annual influenza vaccination remains the most effective prevention strategy and should be recommended for all persons ≥6 months of age without contraindications 1, 6. Oseltamivir is not a substitute for vaccination 6.