What is the recommended treatment for Influenza (flu) A vs Influenza (flu) B?

Treatment of Influenza A vs Influenza B

Primary Recommendation

Both influenza A and influenza B should be treated with the same neuraminidase inhibitors (oseltamivir or zanamivir) using identical dosing regimens, though oseltamivir appears less effective against influenza B compared to influenza A. 1

Core Treatment Approach

First-Line Agent: Oseltamivir

• Oseltamivir 75 mg orally twice daily for 5 days is the preferred treatment for both influenza A and B in adults and adolescents ≥13 years 2, 3, 4
• Initiate treatment within 48 hours of symptom onset for maximal benefit, though earlier initiation (within 36 hours) provides progressively greater symptom reduction 1, 5
• Treatment started within 12 hours of fever onset reduces illness duration by 3.1 days (41%) more than treatment started at 48 hours 5

Alternative Agent: Zanamivir

• Zanamivir 10 mg (two 5-mg inhalations) twice daily for 5 days is an acceptable alternative for patients ≥7 years who cannot tolerate oral oseltamivir 2, 6
• Do not use zanamivir in patients with underlying respiratory disease (asthma, COPD) due to risk of serious bronchospasm 6

Critical Difference: Influenza A vs B Response

Oseltamivir demonstrates reduced efficacy against influenza B compared to influenza A, though it remains the recommended treatment for both. 1

• An observational study in Japanese children showed that those with influenza A resolved fever and stopped viral shedding significantly faster than children with influenza B when treated with oseltamivir 1
• Despite this difference, both zanamivir and oseltamivir have documented activity against influenza B viruses in clinical studies 1
• The same dosing regimen should be used regardless of influenza type—do not increase the dose or duration for influenza B 2, 3

Pediatric Dosing (Weight-Based for Both Influenza A and B)

Children ≥12 months:

• ≤15 kg: 30 mg twice daily 2, 3

• 15-23 kg: 45 mg twice daily 2, 3

• 23-40 kg: 60 mg twice daily 2, 3

• 40 kg: 75 mg twice daily 2, 3

Infants 9-11 months:

• 3.5 mg/kg per dose twice daily 2, 3

Term infants 0-8 months:

• 3 mg/kg per dose twice daily 2, 3

High-Priority Populations Requiring Treatment (Regardless of Influenza Type)

Initiate antiviral treatment immediately in these groups, even without laboratory confirmation during influenza season: 1

• Hospitalized patients with confirmed or suspected influenza 1
• Patients with severe, progressive, or complicated illness (pneumonia, respiratory failure) 1
• Children <2 years 1
• Adults ≥65 years 1
• Pregnant and postpartum women (within 2 weeks after delivery) 1
• Immunocompromised patients 1
• Patients with chronic pulmonary, cardiovascular, renal, hepatic, hematological, metabolic, or neurologic conditions 1

Treatment Beyond 48 Hours

For severely ill or hospitalized patients, initiate oseltamivir even if >48 hours from symptom onset—mortality benefit has been demonstrated up to 96 hours after illness onset. 1

• A prospective study of 754 hospitalized adults showed improved survival when oseltamivir was administered within 4 days of illness onset 1
• Observational data demonstrated that oseltamivir treatment was associated with an 82% reduction in 15-day mortality (OR 0.21) even when started >48 hours after symptom onset 1
• A randomized trial in Bangladesh showed oseltamivir reduced symptom duration and viral shedding even when started ≥48 hours after illness onset 7

Extended Treatment Duration

• Standard duration is 5 days for both influenza A and B 2, 3, 4
• Consider longer treatment (10 days) in immunocompromised patients or those with persistent fever after 6 days, though this remains controversial 1, 3
• Some centers use higher doses (150 mg twice daily) in immunocompromised patients, though evidence is mixed 1

Renal Dose Adjustment (Same for Both Influenza Types)

• For creatinine clearance <30 mL/min, reduce oseltamivir to 75 mg once daily 2, 3
• Oseltamivir is not recommended for end-stage renal disease patients not undergoing dialysis 4

Common Pitfalls to Avoid

Do Not Delay Treatment

• Never wait for laboratory confirmation in high-risk patients during influenza season—rapid tests have poor sensitivity 8
• Negative rapid influenza tests should not exclude treatment in high-risk populations 8

Agents That Do NOT Work Against Influenza B

• Amantadine and rimantadine have NO activity against influenza B and should never be used 2, 3
• These M2 inhibitors only work against influenza A, and even then, resistance rates are extremely high 3

Adverse Effects Management

• Nausea and vomiting occur in 10-15% of patients taking oseltamivir 2, 3, 8
• Taking oseltamivir with food significantly reduces gastrointestinal side effects 8, 9
• These effects are typically transient and occur primarily with the first dose 9, 5

Prophylaxis Considerations (Identical for Both Influenza Types)

Post-Exposure Prophylaxis:

• Oseltamivir 75 mg once daily for 7-10 days, initiated within 48 hours of exposure 2, 3, 8
• Efficacy: 68-89% in household contacts 1, 8

Seasonal Prophylaxis:

• Oseltamivir 75 mg once daily for up to 6 weeks during community outbreaks 3, 8
• Protective efficacy >70% in unvaccinated healthy adults 8, 9

Clinical Outcomes Data

Symptom Duration Reduction:

• Oseltamivir reduces illness duration by 1-1.5 days (approximately 24 hours) when started within 48 hours 3, 8, 9
• Illness severity reduced by up to 38% compared to placebo 8, 9

Complication Reduction:

• Pneumonia risk reduced by approximately 50% 8
• Otitis media incidence decreased by 34% in children 8
• Secondary bacterial infections requiring antibiotics reduced significantly 9

Mortality Benefit:

• Hospitalized patients: 82% reduction in in-patient death for influenza B (OR 0.18) 2
• 15-day mortality reduced by 79% (OR 0.21) in hospitalized adults, even with late treatment 1

Oseltamivir is not a substitute for annual influenza vaccination, which remains the primary prevention strategy. 1, 8, 4, 6