Influenza

Respiratory / Viral

Patient + scenario modifiers

Patient

Clinical scenario / source

Annotations only — chart still shows the full chemistry of each drug.

1. Clinical Syndrome

Acute respiratory illness caused by influenza A (H1N1, H3N2 currently) or influenza B viruses. Classic presentation: abrupt fever + myalgia + headache + cough + sore throat + fatigue. Seasonal peak in temperate climates Oct–Apr. Pediatric + elderly + immunocompromised + pregnant + chronic comorbid disease at highest risk for complications.

Severity spectrum:

Outpatient uncomplicated — symptomatic, no hypoxia, can manage at home.
Hospitalized — hypoxia, hemodynamic compromise, severe comorbidity decompensation, high-risk groups requiring observation.
Severe / ICU — ARDS, hemodynamic instability, secondary bacterial superinfection (S. aureus pneumonia, S. pneumoniae), multi-organ dysfunction.

Excludes: non-influenza ILI (RSV, COVID, parainfluenza, adenovirus — similar clinical, different management), avian influenza (H5N1, H7N9 — separate pandemic-preparedness workflow), influenza-associated bacterial superinfection management (covered under CAP / HAP).

2. Pathogens

Consider the patient: High-risk groups (treat regardless of timing): age ≥65, age <2, pregnant + 2 weeks postpartum, BMI ≥40, immunocompromised (HIV, transplant, malignancy, chronic steroids), chronic lung disease (asthma, COPD), chronic heart disease, diabetes, chronic kidney disease, hematologic disease (sickle cell), neurologic / neurodev disabilities, residents of long-term care, indigenous populations. Lower-risk outpatient: healthy adults with no comorbidity.

Consider the case: Timing from onset (treat within 48 h for max outpatient benefit; treat ANY timing in high-risk / hospitalized / severe), severity (hypoxia → admit + remdesivir/oseltamivir; uncomplicated → outpatient oseltamivir), bacterial superinfection clues (biphasic illness, persistent fever, new lobar infiltrate — empiric CAP + MRSA coverage).

Common

- Influenza A
  H1N1, H3N2 currently circulating; H5N1 + H7N9 avian — pandemic potential.
- Influenza B
  Victoria lineage predominantly. Generally milder than Flu A; can be severe in children + young adults.

Less common

- Staphylococcus aureus (MSSA)
  Bacterial pneumonia superinfection — biphasic illness after initial improvement.
- Staphylococcus aureus (MRSA)
  Necrotizing pneumonia post-influenza — high mortality. PVL-positive community MRSA classic.
- Streptococcus pneumoniae
  Most common bacterial pneumonia post-influenza.

3. Empiric Therapy

Tier	First choice	Alternatives	Duration	Comments
Outpatient	75 mg · PO BID × 5 days · PO	40 mg (<80 kg) or 80 mg (≥80 kg) · PO × 1 dose · PO Single-dose alternative. Avoid in pregnancy (oseltamivir preferred). Don't take with dairy / Ca / Fe / Mg / Zn — chelation kills absorption.	5 days (uncomplicated outpatient)	Start within 48 h of symptom onset for max benefit (reduces duration ~1 day). Outside 48 h: low yield in healthy adults, but TREAT in high-risk groups regardless of timing. Tested vs presumed flu: rapid POC molecular flu testing (where available) increases targeted treatment; otherwise treat empirically during peak season when clinical picture fits.
Admitted to ward	75 mg · PO BID × 5–10 days (extend if severe / immunocompromised / persistent viral shedding) · PO	40–80 mg PO × 1 dose · — · PO Limited data in hospitalized; oseltamivir preferred standard.	5–10 days (hospitalized); extend if immunocompromised + persistent shedding	Hospitalized / severe / immunocompromised — treat regardless of symptom-onset timing. High-dose oseltamivir (150 mg BID) considered in severe / immunocompromised per IDSA 2018 (limited data; mortality benefit not clearly proven). Evaluate + treat empirically for bacterial superinfection if persistent fever, new lobar infiltrate, hemoptysis — cover MSSA + MRSA + S. pneumoniae (CAP + vanc empirics).
Admitted to ICU	75–150 mg · PO BID × 10 days · PO Via NG/OG tube if intubated. Higher dose considered in severe disease (modest evidence).	—	10 days minimum in severe disease; extend if viral shedding persists	Severe ARDS, sepsis, multi-organ dysfunction. Add empiric bacterial coverage (CAP + MRSA) until ruled out. Lung-protective ventilation, prone positioning, ECMO consideration in severe ARDS. Steroids generally NOT helpful for severe influenza unless concurrent ARDS protocol; some data showing harm in pure flu ARDS.

Add coverage if:

MRSA coverage

Necrotizing pneumonia post-flu
Severe / hospitalized influenza with bacterial superinfection clues
Cavitary infiltrate

Add:

load + AUC-guided · — · IV
**Post-influenza S. aureus pneumonia** is a documented + devastating association — empiric vanc + clinda for toxin suppression (PVL).

4. Directed Therapy

Antivirals are class-directed (NA inhibitors for A + B; baloxavir for both). Switching usually unnecessary except resistance suspicion.

Post-exposure prophylaxis (in high-risk close contacts within 48 h): oseltamivir 75 mg PO daily × 7 days, or baloxavir × 1 dose. Vaccination is the primary prevention.

Vaccination (annual): inactivated quadrivalent or recombinant for most adults; high-dose / adjuvanted for ≥65; live attenuated nasal (FluMist) for healthy ages 2–49 (NOT pregnant or immunocompromised).

5. Monitoring

Resolution: fever resolves typically 3–5 days; cough + fatigue 1–2 weeks. Persistent fever > 5–7 days or biphasic (improving then worsening) → bacterial superinfection workup (chest imaging, blood + sputum cultures, repeat exam).

Severe disease: SpO2, work of breathing, mental status, lactate. Repeat CXR if worsening.

Pearls

Treat hospitalized + severe + high-risk regardless of symptom-onset timing. Outpatient mild disease: 48-h window for max benefit. Bacterial superinfection is the killer — biphasic illness, lobar infiltrate, hemoptysis → think S. aureus (incl MRSA) or pneumococcus. Annual vaccination is the primary prevention; high-dose / adjuvanted for ≥65. Baloxavir single-dose is the adherence win — but watch the dairy/calcium interaction. Don't use adamantanes (amantadine, rimantadine) — universal resistance.