What influenza virus strain is currently dominant in Melbourne and what are the recommended vaccine and antiviral treatments?

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Last updated: April 10, 2026 • View editorial policy

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Current Influenza Strain in Melbourne and Treatment Recommendations

Based on 2022 surveillance data from the Melbourne WHO Collaborating Centre, influenza A(H3N2) viruses predominated, accounting for 77% of all analyzed viruses, with the majority being antigenically similar to WHO-recommended vaccine strains and susceptible to neuraminidase inhibitors 1.

Dominant Circulating Strain

In Melbourne during 2022, influenza A(H3N2) subclade 3C.2a1b.2a.2 was the dominant strain, with viruses clustering with the 2022 vaccine strain 1, 2. The genetic analysis revealed that Victoria (the state where Melbourne is located) drove A(H3N2) transmission across Australia during this season 2.

However, a critical update for 2025: Subclade K viruses emerged late in the 2025 Australian season, extending the influenza season. These viruses are genetically and antigenically distinct from the 2025 vaccine A(H3N2) strain and have since spread to over 34 countries globally 3.

Recommended Vaccine Composition

For the 2025-2026 season, all licensed vaccines in the United States are trivalent 4:

  • H1N1: A/Victoria/4897/2022 (H1N1)pdm09-like virus (unchanged)
  • H3N2: A/Croatia/10136RV/2023 (H3N2)-like virus (NEW this season)
  • Influenza B: B/Austria/1359417/2021-like virus (Victoria lineage, unchanged)

Annual influenza vaccination is strongly recommended for all children ≥6 months of age, with particular emphasis on high-risk groups including children <5 years (especially <2 years), those with chronic medical conditions, and immunocompromised children 4, 5.

Antiviral Treatment Recommendations

For Non-Severe Influenza

Oseltamivir (Tamiflu) 75 mg twice daily for 5 days is the first-line antiviral treatment for adults and children when initiated within 48 hours of symptom onset 6, 7, 8.

Pediatric dosing for oseltamivir 9:

  • Children >40 kg: 75 mg twice daily
  • 23-40 kg: 60 mg twice daily
  • 15-23 kg: 45 mg twice daily
  • ≤15 kg: 30 mg twice daily
  • Infants 3-12 months: 3 mg/kg/dose twice daily
  • Infants <3 months: 3 mg/kg/dose twice daily (use only if situation judged critical)

Dose adjustment: Reduce by 50% if creatinine clearance <30 mL/minute 6, 10.

Baloxavir is conditionally recommended for patients at high risk of progression to severe illness 8, 11. This includes children with chronic pulmonary disease (including asthma), cardiovascular disease, metabolic disorders, neurologic conditions, or immunosuppression 4, 5.

For Severe Influenza

For hospitalized patients with severe influenza, oseltamivir should be initiated immediately, even if >48 hours from symptom onset 6, 7. Consider:

  • Standard dose: 75 mg twice daily for 5 days
  • Extended duration: 10 days or until symptom resolution in immunocompromised patients 7
  • Higher doses (150 mg twice daily) have been used in some immunocompromised patients with mixed results 7

Alternative antivirals:

  • Zanamivir 10 mg (two 5-mg inhalations) twice daily - avoid in patients with chronic respiratory disease due to bronchospasm risk 9, 10
  • Peramivir (IV) - for patients unable to absorb oral oseltamivir or tolerate zanamivir 7

Antiviral Resistance

Antiviral susceptibility testing from Melbourne 2022 showed excellent sensitivity: Of 3,372 samples tested, only two A(H1N1)pdm09 viruses showed highly reduced inhibition against oseltamivir 1. Current circulating strains remain sensitive to neuraminidase inhibitors (oseltamivir, zanamivir, peramivir).

Do NOT use amantadine or rimantadine - circulating influenza A viruses have extremely high resistance levels, and these drugs are ineffective against influenza B 10, 12.

Chemoprophylaxis Recommendations

Post-exposure prophylaxis is conditionally recommended for 4, 8, 11:

  • Unimmunized high-risk children during the 2 weeks after vaccination
  • Unimmunized household contacts of high-risk individuals
  • Asymptomatic persons exposed to seasonal influenza who are at extremely high risk of hospitalization

Prophylaxis dosing 9:

  • Adults: Oseltamivir 75 mg once daily for 10 days
  • Children: Same weight-based dosing as treatment, but once daily instead of twice daily

Antibiotic Use

Strong recommendation AGAINST routine antibiotics for influenza not complicated by pneumonia 6, 13.

Consider antibiotics only when:

  • Recrudescent fever or increasing dyspnea develops
  • High-risk patients develop lower respiratory features
  • Influenza-related pneumonia is present

Preferred antibiotics for influenza-related pneumonia:

  • Non-severe: Co-amoxiclav or tetracycline (oral)
  • Severe: IV co-amoxiclav or 2nd/3rd generation cephalosporin PLUS macrolide (clarithromycin or erythromycin) 6, 13

Critical Clinical Pitfalls

  1. Do not wait for test results to initiate treatment - rapid antigen tests have low sensitivity, particularly for H1N1, and negative results should not rule out influenza 9

  2. Do not withhold antivirals from elderly or immunocompromised patients lacking documented fever - they may be unable to mount adequate febrile response but still benefit from treatment 6, 13

  3. Treatment benefit is highest within 48 hours, but hospitalized severely ill patients may benefit even after this window 6, 7, 13

  4. Vaccination coverage in Australia remains suboptimal - only 53.9% of children were vaccinated in the 2023-2024 season, with significant disparities by race, ethnicity, and geography 5

References

3
Research

Extended influenza seasons in Australia and New Zealand in 2025 due to the emergence of influenza A(H3N2) subclade K viruses.

Euro surveillance : bulletin Europeen sur les maladies transmissibles = European communicable disease bulletin, 2025

8
Research

[WHO clinical practice guidelines for influenza: an update].

Gesundheitswesen (Bundesverband der Arzte des Offentlichen Gesundheitsdienstes (Germany)), 2025

11
Research

Summary of WHO clinical practice guidelines for influenza.

BMJ (Clinical research ed.), 2026

12
Research

Influenza.

Nature reviews. Disease primers, 2018

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.