Influenza (Flu): Symptoms, Treatment, and Prevention Guide

Understanding Influenza

Influenza (commonly called "flu") is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. Unlike many other viral respiratory infections, influenza can cause severe illness and life-threatening complications, particularly in high-risk groups.

Each year, influenza causes:

• Millions of illnesses (9-41 million in the U.S. annually)
• Hundreds of thousands of hospitalizations (100,000-700,000 in the U.S.)
• Thousands of deaths (12,000-52,000 in the U.S.)

The virus mutates constantly, creating new strains each year. This is why seasonal flu vaccines are updated annually and why you can get the flu more than once in your lifetime.

Recognizing Flu Symptoms

Classic Influenza Presentation

Sudden onset (patients often recall exact time symptoms started):

• High fever (100-104°F/38-40°C) lasting 3-5 days
• Chills, body aches, severe fatigue
• Dry cough, sore throat
• Headache, nasal congestion
• Post-viral fatigue: Weakness can last 2+ weeks

Flu vs Cold Comparison

| Symptom | Cold | Flu | |---------|------|-----| | Onset | Gradual (days) | Abrupt (hours) | | Fever | Rare/uncommon | Common, high (100-104°F) | | Headache | Rare | Common, often severe | | Body aches | Mild | Severe | | Fatigue | Mild | Severe, can last weeks | | Cough | Mild to moderate | Common, often severe | | Sneezing | Common | Less common | | Sore throat | Common | Common |

Complications Risk Factors

High-risk for complications:

• Age ≥65 years or ＜5 years

• Pregnancy (up to 2 weeks postpartum)

• Chronic medical conditions:

  • Lung disease (asthma, COPD)
  • Heart disease
  • Diabetes
  • Kidney/liver disease
  • Weakened immune system
  • Obesity (BMI ≥40)
  • Neurologic/neuromuscular conditions

• Residents of nursing homes/long-term care facilities

Complications include:

• Pneumonia (flu-related or bacterial superinfection)
• Bronchitis, sinus infections, ear infections
• Worsening of chronic medical conditions
• Myocarditis, encephalitis (rare but serious)
• Death (highest in those with risk factors)

Diagnosis

Clinical Diagnosis

Influenza-Like Illness (ILI) Definition:

• Fever ≥100°F (37.8°C) AND
• Cough AND/OR sore throat
• In the absence of other known cause

During flu season when influenza is circulating, clinical diagnosis is reasonably accurate (~70%) in patients with typical symptoms.

Laboratory Testing

Rapid Influenza Diagnostic Tests (RIDTs):

• Results in 10-15 minutes
• Nasal/throat swab
• Sensitivity: 50-70% (misses many cases)
• Positive result is accurate (high specificity), negative doesn't rule out flu

Rapid Molecular Assays:

• Results in 15-30 minutes
• Higher sensitivity (＞90%) than antigen tests
• More expensive than RIDTs

RT-PCR (Polymerase Chain Reaction):

• Gold standard (sensitivity ＞95%)
• Results in hours to days
• Used for surveillance, hospitalized patients

When to Test:

• Hospitalized patient with suspected flu
• High-risk patient with flu-like symptoms
• Outbreak investigation (nursing homes, schools)
• When diagnosis will guide management (antiviral use)

When NOT to Test:

• Healthy person with mild illness not requiring hospitalization (treat empirically if within treatment window)

Treatment Strategies

Symptomatic Treatment

Fever and Pain:

• Acetaminophen (Tylenol): 650-1000 mg q6h PRN (max 3000 mg/day)
• Ibuprofen (Advil, Motrin): 400-600 mg q6h PRN (max 2400 mg/day)
• Alternate acetaminophen/ibuprofen for high fever
• Avoid aspirin in children/teens (Reye syndrome risk)

Hydration:

• Clear fluids: Water, broth, electrolyte solutions (Pedialyte)
• Avoid: Alcohol, excessive caffeine (dehydrating)
• Goal: Pale yellow urine (good hydration)

Rest:

• Stay home until fever-free for 24 hours (without fever-reducing meds)
• Return to normal activities gradually
• Exercise: Wait until fully recovered (post-viral fatigue common)

Antiviral Medications

When to Use Antivirals (within 48 hours of symptom onset):

Recommended for:

• Hospitalized with suspected/confirmed flu
• High-risk outpatients with flu-like illness
• Anyone with severe/progressive illness
• Outpatients who are not high-risk BUT want to shorten illness

Benefits of Antivirals:

• Shortens illness by 1-2 days
• Reduces complications risk (hospitalization, death) by 30-50%
• Reduces viral shedding (less contagious)

CDC-Recommended Antivirals:

| Medication | Dosing | Duration | Side Effects | Efficacy | |------------|--------|----------|--------------|----------| | Oseltamivir (Tamiflu) | 75 mg BID | 5 days | Nausea, vomiting | Shortens illness 1-2 days | | Baloxavir (Xofluza) | Single dose | 1 dose | Diarrhea, nausea | Shortens illness 1 day | | Zanamivir (Relenza) | 10 mg BID (inhaled) | 5 days | Bronchospasm (asthma) | Similar to oseltamivir | | Peramivir (Rapivab) | IV single dose | 1 dose | Diarrhea | Hospitalized patients |

Oseltamivir (Tamiflu) - Most Commonly Prescribed:

• Timing: Most effective within 48 hours, but consider up to 5 days in hospitalized/high-risk
• Children: Dosing based on weight
• Side effects: Nausea (take with food), vomiting, headache
• Resistance: Currently low but can develop

Baloxavir (Xofluza):

• Single oral dose (convenient)
• Not for pregnancy, breastfeeding, or hospitalized patients
• Not for children ＜5 years or weight ＜20 kg
• More expensive than oseltamivir

Special Populations

Pregnancy:

• High-risk for severe flu and complications
• Oseltamavir preferred (most safety data)
• Treat aggressively even if ＞48 hours from symptom onset
• Prompt treatment reduces hospitalization, preterm birth

Children:

• Increased risk for complications ＜5 years, especially ＜2 years
• Avoid aspirin (Reye syndrome: liver/brain damage)
• Consider antivirals for any child with flu-like symptoms
• Monitor for dehydration

Adults ≥65 years:

• Highest hospitalization and death rates
• Antivirals recommended regardless of symptom duration
• High-dose flu vaccine preferred for prevention

Immunocompromised:

• Prolonged viral shedding (weeks to months)
• Consider extended antiviral course (10+ days)
• Higher risk for antiviral resistance (oseltamivir-resistant strains)

Prevention Strategies

Vaccination (Primary Prevention)

Annual Flu Vaccine:

• Recommended for: Everyone ≥6 months (with rare exceptions)
• Best timing: September-October (before flu season peaks)
• Effectiveness: Varies 40-60% depending on season, vaccine match
• Duration: Protection lasts ~6 months (need annual vaccine)

Vaccine Types:

| Type | Who Should Get | Notes | |------|----------------|-------| | Standard flu shot | Most people 6+ months | Inactivated virus, cannot cause flu | | High-dose shot | Adults ≥65 years | 4x antigen, better immune response | | Adjuvanted shot | Adults ≥65 years | Boosts immune response | | Nasal spray | Non-pregnant 2-49 years | Live attenuated, avoid in immunocompromised | | Egg-free options | Egg allergy | Recombinant, cell-based vaccines |

Vaccine Safety:

• Cannot cause flu (inactivated virus)
• Common side effects: Soreness, redness at injection site, low-grade fever, aches (last 1-2 days)
• Severe allergic reactions: Rare (＜1/1,000,000)
• GBS (Guillain-Barré Syndrome): Very rare (1-2 per million), flu itself much higher risk

Who Should NOT Get Flu Vaccine:

• Severe allergic reaction to previous flu vaccine
• ＜6 months old
• Moderate/severe acute illness (wait until recovered)
• Egg allergy alone is NOT a contraindication (egg-free options available)

Non-Pharmacologic Prevention

Hand Hygiene:

• Wash hands frequently with soap/water (20 seconds)
• Alcohol-based hand sanitizer if soap unavailable
• Avoid touching eyes, nose, mouth

Respiratory Etiquette:

• Cover cough/sneeze with tissue or elbow
• Wear mask if sick or caring for sick person
• Stay home when sick (until fever-free 24 hours)

Environmental Measures:

• Clean frequently-touched surfaces (doorknobs, phones, remotes)
• Improve ventilation (open windows, air purifiers)
• Avoid close contact with sick people

Healthy Habits:

• Adequate sleep (7-9 hours)
• Regular physical activity
• Manage stress
• Healthy diet (vitamin C, D, zinc may modestly help)

Post-Influenza Care

Recovery Timeline

Acute symptoms: 5-7 days

• Fever: Usually resolves by day 3-5
• Respiratory symptoms: Cough may persist 2+ weeks
• Fatigue: Can last 1-3 weeks (longer in older adults)

Return to Normal Activities:

• Work/school: After fever-free 24 hours WITHOUT fever-reducing meds
• Exercise: Wait until fully recovered (typically 1-2 weeks)
• Travel: When recovered, avoid spreading illness

Post-Viral Complications

Bacterial Superinfection:

• Symptoms improve then return with fever, worsening cough
• Pneumonia, sinusitis, ear infections
• Requires antibiotics (not for initial flu)

Post-Viral Fatigue:

• Persistent weakness lasting weeks
• Gradual return to normal activities
• Rest, hydration, time

Exacerbation of Chronic Conditions:

• Asthma/COPD flares
• Heart failure exacerbation
• Diabetes dysregulation
• Monitor closely if you have chronic diseases

Related Conditions

• COVID-19: Similar respiratory illness, more severe
• Pneumonia: Common flu complication
• RSV: Another respiratory virus, similar symptoms

FAQ

References