First-Line Treatment for H3N2 Influenza
Oseltamivir 75 mg orally twice daily for 5 days is the first-line treatment for H3N2 influenza, initiated as soon as possible after diagnosis, ideally within 48 hours of symptom onset. 1
Primary Treatment Recommendation
Oseltamivir is the preferred neuraminidase inhibitor for H3N2 treatment because H3N2 strains remain susceptible to neuraminidase inhibitors while demonstrating widespread resistance to adamantanes (amantadine and rimantadine). 1, 2
The standard adult dose is 75 mg orally twice daily for 5 days. 1
For pediatric patients ≥13 years, the same adult dosing applies (75 mg twice daily for 5 days). 1
For children <13 years, weight-based dosing is required, varying by age and weight. 1
Alternative Agent
Zanamivir 10 mg (2 inhalations) twice daily for 5 days is an acceptable alternative for patients without chronic respiratory disease. 1, 2
Zanamivir should NOT be used in patients with underlying airway disease (asthma, COPD) due to risk of life-threatening bronchospasm. 1, 3
Critical Timing Considerations
Maximum benefit occurs when treatment is initiated within 48 hours of symptom onset. 1, 2
However, for hospitalized patients or those with severe illness, treatment should be initiated regardless of time since symptom onset (even beyond 48 hours). 1, 2
Early treatment in severely ill patients, even up to <5 days after onset, has been associated with reduced morbidity and mortality in observational studies. 2
High-Priority Treatment Groups
Treatment should be initiated immediately upon diagnosis for all patients in high-risk categories, including: 1, 2
- Children <2 years of age
- Adults ≥65 years
- Pregnant women and postpartum women (within 2 weeks after delivery)
- Patients with chronic pulmonary disease (including asthma)
- Patients with cardiovascular disease (except hypertension alone)
- Patients with chronic metabolic disorders (including diabetes)
- Patients with immunosuppression
- Patients requiring hospitalization
Agents NOT Recommended
Adamantanes (amantadine and rimantadine) should NOT be used for H3N2 treatment due to widespread resistance among circulating H3N2 strains. 1, 2, 4
During the 2012-2013 season, high levels of resistance to adamantanes existed, making these drugs ineffective. 2
This resistance pattern has persisted, and adamantanes remain inappropriate for H3N2 treatment. 1
Common Pitfalls to Avoid
Do not delay treatment while waiting for laboratory confirmation in high-risk patients during influenza season—treat empirically based on clinical suspicion. 1, 2
Do not withhold treatment beyond 48 hours for hospitalized patients, as benefit has been demonstrated even with later initiation. 1, 2
Do not use zanamivir in patients with underlying airway disease due to bronchospasm risk. 1
Do not assume rapid antigen tests are definitive—negative tests do not rule out influenza, and treatment decisions should be based on clinical judgment and local surveillance data. 2
Post-Exposure Prophylaxis
For exposed individuals, oseltamivir 75 mg once daily for 10 days can be used for post-exposure prophylaxis if initiated within 48 hours of exposure. 1
Prophylaxis is not a substitute for vaccination and should be reserved for specific high-risk situations. 2
Monitoring and Surveillance
Consult CDC surveillance data routinely for current resistance patterns, as antiviral susceptibility can change rapidly from season to season. 1, 2
Monitor for treatment failure, which may indicate antiviral resistance and warrant consideration of alternative therapy or consultation with infectious disease specialists. 1
Among recent H3N2 viruses tested, resistance to oseltamivir remains extremely low (0.05%), supporting its continued use as first-line therapy. 2