H1N1 (Swine Flu) Symptoms: Incubation, Contagious Period & What to Expect

H1N1 Influenza: Timeline & Clinical Picture

Incubation period: 1–4 days (average 2 days); range 1–7 days.

Contagious period: 1 day before symptom onset → 5–7 days after onset (longer in children, immunocompromised).

Typical Symptom Onset & Progression

Phase	Timeline	Features
Prodrome	Day 0–1	Sudden onset: fever/chills, myalgia, headache, fatigue
Respiratory phase	Day 1–3	Dry cough, sore throat, nasal congestion, dyspnea
GI symptoms	Variable	Nausea, vomiting, diarrhea (more common in children than seasonal flu)
Peak severity	Day 2–4	High fever (39–40°C), severe myalgia, prostration
Recovery	Day 5–10	Fever resolves; cough/fatigue may persist 2+ weeks

Red Flags: Seek Immediate Care

• Difficulty breathing, chest pain, cyanosis
• Altered mental status, confusion, seizures
• Persistent high fever >3 days
• Dehydration (oliguria, dizziness, dry mucous membranes)
• Worsening after initial improvement (secondary bacterial pneumonia)
• High-risk groups: pregnant, <5 or >65 years, chronic diseases (asthma, COPD, diabetes, heart disease, immunosuppression)

Diagnosis & Treatment

• Rapid influenza diagnostic tests (RIDT): 10–15 min, moderate sensitivity (50–70%).
• RT-PCR: Gold standard; distinguishes H1N1 from other strains.
• Antivirals (oseltamivir, zanamivir, peramivir, baloxavir): Most effective if started ≤48 hours of onset. Reduce duration by ~1 day, lower complications. Recommended for high-risk/hospitalized patients.
• Supportive care: Hydration, antipyretics (avoid aspirin in <18 years), rest, isolation.

Prevention

• Annual flu vaccine (includes H1N1 component since 2009).
• Hand hygiene, respiratory etiquette, masking in high-risk settings.
• Post-exposure prophylaxis (antivirals) for high-risk close contacts.

H1N1 is now a seasonal influenza strain. Most cases are self-limited, but vigilance for complications—especially in high-risk groups—is essential.