influenza and COVID-19
in review
3 min read
Updated 2026-09-29
influenza and COVID-19
Influenza: oseltamivir 75 mg BID × 5 days for all hospitalised patients (regardless of symptom duration), severe/progressive illness, and high-risk outpatients — best within 48 h but still given later. Watch for secondary S. aureus/pneumococcal pneumonia. COVID-19: high-risk outpatients → nirmatrelvir/ritonavir within 5 days (check interactions) or 3 days of remdesivir within 7 days; hospitalised on oxygen → dexamethasone 6 mg × 10 days (+ remdesivir on low-flow); escalating oxygen → add tocilizumab or baricitinib. No dexamethasone without an oxygen requirement.
do not miss
- avian influenza (H5N1) — poultry, wild bird, or dairy cattle exposure + influenza-like illness or conjunctivitis → airborne + contact + droplet precautions, oseltamivir, notify public health/IPAC
- post-influenza necrotising S. aureus (incl. MRSA) pneumonia — rapid deterioration after initial improvement → vancomycin or linezolid
influenza
who to treat (AMMI Canada 2025–26)
- hospitalised — always, regardless of time since onset
- severe, complicated, or progressive illness
- outpatients at higher risk: age ≥65, pregnancy/≤2 weeks postpartum, chronic cardiac/pulmonary/renal/hepatic disease, diabetes, immunosuppression, obesity (BMI ≥40), neurological disease affecting airway clearance, long-term care residents, Indigenous peoples
- healthy outpatients with mild illness: not routinely
agents
| agent | dose | notes |
|---|---|---|
| oseltamivir | 75 mg PO BID × 5 days | preferred for hospitalised patients and pregnancy; renal dose reduction; no benefit from double dose; consider 10 days if immunocompromised with ongoing shedding |
| baloxavir | single weight-based dose (≥12 years, ≥40 kg) | outpatients; limited data in severe illness; resistance emergence; not studied in pregnancy |
| zanamivir | 10 mg inhaled BID × 5 days | oseltamivir-resistant strains; avoid in asthma/COPD (bronchospasm) |
| IV options (peramivir, IV zanamivir) | — | Special Access Programme only — no enteral route or resistance |
- chemoprophylaxis in long-term care outbreaks: oseltamivir 75 mg daily for residents (and unvaccinated staff) until the outbreak is declared over — per public health
- testing: PCR (multiplex with SARS-CoV-2/RSV) in admitted patients; do not delay treatment for results in high-risk patients during influenza season
complications
- secondary bacterial pneumonia (S. pneumoniae, S. aureus, GAS) — biphasic course
- myositis/rhabdomyolysis, myocarditis, encephalopathy, exacerbations of COPD/asthma/heart failure
COVID-19
outpatients at high risk (within the first days)
| agent | window | notes |
|---|---|---|
| nirmatrelvir/ritonavir (Paxlovid) × 5 days | ≤5 days of symptoms | ritonavir CYP3A4 interactions — statins, DOACs, amiodarone, tacrolimus, some antiepileptics, sildenafil; renal dose adjustment |
| remdesivir IV × 3 days | ≤7 days | when interactions prohibit Paxlovid |
High risk: older age (≥60–65), immunocompromise, unvaccinated/under-vaccinated with comorbidities, pregnancy.
hospitalised
| severity | treatment |
|---|---|
| no oxygen requirement | remdesivir if high risk of progression; no dexamethasone (possible harm) |
| low-flow oxygen | dexamethasone 6 mg daily × 10 days (or to discharge) + remdesivir 5 days |
| high-flow / NIV | dexamethasone + tocilizumab or baricitinib |
| invasive ventilation | dexamethasone + tocilizumab or baricitinib |
- thromboprophylaxis for all hospitalised patients
- persistent or relapsing COVID in profound immunosuppression (B-cell depletion) → ID input (prolonged/combination antivirals)
avian influenza (H5N1)
- exposures: sick/dead poultry or wild birds, infected dairy cattle (raw milk), mammals; human cases reported in North America including Canada
- clinical: conjunctivitis to severe pneumonia/ARDS
- isolate (airborne + contact + droplet, eye protection) → notify public health and IPAC before specimen transport
- oseltamivir 75 mg BID promptly (duration per public health; often longer in severe disease)
traps
- dexamethasone for COVID without an oxygen requirement
- withholding oseltamivir from a hospitalised patient because symptoms started >48 h ago
- Paxlovid without an interaction review (tacrolimus toxicity, statin myopathy, DOAC bleeding)
- zanamivir inhalation in asthma or COPD
- treating a biphasic post-influenza deterioration as “viral” — cover S. aureus