Fellow's quick read · Infectious Diseases
Seasonal influenza
IDSA 2018 Update · reviewed 2026-07-05
Personal study digest for a new ID fellow. Recommendations are drawn from the IDSA 2018 seasonal influenza guideline (Uyeki et al, CID 2019;68:e1-e47). The "What's changed since 2018" section is reviewer synthesis of newer evidence, each claim cited. Not a substitute for the full guideline.
In one line
Do not let diagnostic testing delay antiviral treatment: start an antiviral empirically in anyone hospitalized, severely ill, or at high risk of complications — regardless of illness duration — and reserve chemoprophylaxis for defined high-risk exposures, not routine use.◆When to suspect / diagnose
- During community influenza activity, test all hospitalized patients with acute respiratory illness (with or without fever), and any patient with acute worsening of chronic cardiopulmonary disease (A-II/A-III).
- Outpatients: test high-risk or immunocompromised patients if the result will change management (A-III); testing in low-risk outpatients is optional and should never delay empiric treatment.
- Atypical presentation is common in the immunocompromised and elderly — a low bar to test in these groups.
◆Workup the fellow drives
- Molecular assay (NAAT/RT-PCR) over rapid antigen tests (RIDTs) in outpatients, and RT-PCR (not RIDT, not immunofluorescence) in hospitalized patients (A-II).
- Multiplex respiratory panel in hospitalized immunocompromised patients (A-III); consider it in other inpatients if it changes cohorting or antibiotic use.
- Do not use viral culture or serology for primary diagnosis — too slow or unreliable to guide management (A-III).
- In a ventilated patient with a negative upper-respiratory swab but ongoing suspicion, test endotracheal aspirate/BAL (A-II).
◆Empiric & definitive therapy
| Scenario | Drug / dose / route | Duration |
|---|---|---|
| Uncomplicated, otherwise healthy | Oseltamivir 75 mg PO BID (weight-based peds) or inhaled zanamivir or single-dose IV peramivir | 5 days (single dose for peramivir) |
| Uncomplicated, ≥5 y, ≤48 h of illness, not pregnant/hospitalized/severely immunocompromised | Baloxavir marboxil — single oral dose, weight-based (40 mg <80 kg, 80 mg ≥80 kg) | Single dose |
| Hospitalized, severe/progressive illness, or high-risk of complications (any age) | Start an NAI (oseltamivir, zanamivir, or peramivir) ASAP regardless of illness duration (A-II/A-III) | Consider >5 d if immunocompromised or severe LRTI (C-III) |
| Postexposure chemoprophylaxis (nonoutbreak) | Oral oseltamivir or inhaled zanamivir, start ≤48 h after exposure | 7 days after most recent exposure |
- Never combine two NAIs (A-1); don't use higher-than-approved NAI doses (A-II).
- Chemoprophylaxis is not for routine/widespread use outside institutional outbreaks — reserve for very-high-risk unvaccinated contacts or severely immunocompromised patients for whom vaccination fails/is unavailable.
◆Key decisions a fellow owns
- Don't wait for the test result to start antivirals in anyone who qualifies for treatment — greatest benefit is within 48 h of symptom onset, but treat hospitalized/high-risk patients regardless of duration.
- Investigate and empirically treat bacterial coinfection in patients who present with severe disease (extensive pneumonia, respiratory failure, hypotension) or who deteriorate after initial improvement (A-II/A-III).
- Do not give adjunctive corticosteroids or IVIG for influenza pneumonia, respiratory failure, or ARDS unless indicated for another reason (A-III, both).
- Consider NAI-resistance testing if a patient develops influenza during/after NAI chemoprophylaxis, has persistent viral replication after 7–10 days while immunocompromised, or fails to improve on treatment despite severe illness.
◆Special populations
- Pregnancy (any trimester): oseltamivir preferred over zanamivir/IV peramivir; some experts use higher dosing (105–150 mg BID) given altered pharmacokinetics — evidence limited.
- Severely immunocompromised: viral shedding is prolonged; treatment can reasonably extend to 10 days; no data support higher NAI doses in this group.
- Baloxavir is NOT recommended for pregnant/breastfeeding women, severely immunocompromised patients, hospitalized patients, or outpatients with complicated/progressive illness — no efficacy/safety data in these groups; oseltamivir remains preferred.
◆Duration & stopping
- Uncomplicated ambulatory illness: 5 days of oseltamivir or zanamivir, or a single peramivir/baloxavir dose.
- Extend beyond 5 days for hospitalized, severely ill, or immunocompromised patients with protracted viral replication (C-III) — no fixed endpoint, guided by clinical course.
What's changed since 2018
Reviewer synthesis of newer evidence — not the guideline. Each claim cited; flags where the 2018 IDSA guideline lags current practice.
- Baloxavir marboxil — an entirely new drug class arrived the same month the guideline was finalized. FDA approved baloxavir (a polymerase-acidic endonuclease inhibitor) on 24 Oct 2018, days before this guideline's online publication, so it is absent from the document. CAPSTONE-1 (healthy 12–64 y) showed a single oral dose shortened symptoms similarly to 5-day oseltamivir with faster viral load decline; CAPSTONE-2 confirmed similar efficacy in high-risk outpatients. Hayden, NEJM 2018;379:913-923 (PMID 30184455) · Ison, Lancet Infect Dis 2020;20:1204-1214 (PMID 32526195)
- Baloxavir now has a defined guardrail, not a guideline gap. CDC does not recommend baloxavir monotherapy for hospitalized patients, severely immunocompromised patients, pregnant/breastfeeding women, or outpatients with complicated/progressive illness — no efficacy/safety data exist in these groups. For exactly the populations the 2018 guideline emphasizes (hospitalized, high-risk), oseltamivir remains the drug of record. Pediatric approval (age ≥5 y, treatment and postexposure prophylaxis) followed in Aug 2022 based on miniSTONE-2 and BLOCKSTONE. CDC Influenza Antiviral Medications: Summary for Clinicians (cdc.gov/flu/hcp/antivirals)
- Single-dose baloxavir also works as postexposure chemoprophylaxis — BLOCKSTONE (household contacts, Japan): clinical influenza developed in 1.9% of baloxavir recipients vs 13.6% of placebo recipients (adjusted RR 0.14). A post hoc analysis further found households of baloxavir-treated index cases had a lower secondary attack rate than households of oseltamivir-treated index cases (10.8% vs 18.5%, adjusted relative reduction 41.8%). The 2018 guideline's chemoprophylaxis section names only oseltamivir/zanamivir. Ikematsu, NEJM 2020;383:309-320 (PMID 32640124) · Ikematsu, Influenza Other Respir Viruses 2024;18:e13302 (PMID 38706384)
- Treatment-emergent reduced susceptibility is baloxavir's known trade-off. PA/I38T (or I38M/F) substitutions conferring reduced baloxavir susceptibility emerge in ~10% of treated patients overall (up to ~23% in some pediatric cohorts), associated with prolonged viral shedding and, uncommonly, symptom rebound — but clinical efficacy is generally preserved because resistance emerges after treatment has already worked. Worth knowing, not yet practice-changing. Uehara, J Infect Dis 2020;221:346-355 (PMID 31309975)
- Oseltamivir/NAI resistance remains uncommon but real and monitored each season. US surveillance for the 2025-2026 season identified a small number of A(H1N1)pdm09 viruses with the NA-H275Y substitution (high-level oseltamivir/peramivir resistance) plus additional viruses with reduced-susceptibility substitutions, and rare A(H3N2) NA-E119V — reinforcing the guideline's existing recommendation (rec 28) to consider resistance testing in persistent or refractory cases. CDC Influenza Antiviral Drug Resistance, 2025-2026 season (cdc.gov/flu/treatment/antiviralresistance.html)
- Still current: no newer IDSA seasonal influenza guideline has been published since 2018 (confirmed via idsociety.org and PubMed) — this remains the operative US reference. The "test everyone hospitalized, treat without waiting for results" framework and the recommendation against adjunctive corticosteroids/IVIG both still stand.
- Open/unresolved: whether adding baloxavir to standard NAI therapy improves outcomes in hospitalized/severe influenza is still being tested (e.g., NCT03684044) — no proven combination-therapy benefit yet, mirroring the 2018 guideline's caution against combining antivirals.
◆Anki cards minted this run
- Baloxavir marboxil — single-dose mechanism/approval, arrived after the 2018 guideline (CAPSTONE-1/2).
- Baloxavir NOT recommended — hospitalized, pregnant, severely immunocompromised, complicated/progressive illness (oseltamivir preferred).
- Corticosteroids NOT recommended for influenza pneumonia/ARDS (contrast with severe non-influenza CAP).
- BLOCKSTONE — single-dose baloxavir postexposure prophylaxis cuts household secondary attack rate.
Sources: Uyeki et al, IDSA 2018 seasonal influenza guideline, CID 2019;68:e1-e47 (doi:10.1093/cid/ciy866); Hayden, NEJM 2018;379:913-923 (PMID 30184455); Ison, Lancet Infect Dis 2020;20:1204-1214 (PMID 32526195); Ikematsu, NEJM 2020;383:309-320 (PMID 32640124); Ikematsu, Influenza Other Respir Viruses 2024;18:e13302 (PMID 38706384); Uehara, J Infect Dis 2020;221:346-355 (PMID 31309975); CDC Influenza Antiviral Medications: Summary for Clinicians; CDC Influenza Antiviral Drug Resistance (2025-2026 season); CDC/Genentech baloxavir approval history (Xofluza, FDA 24 Oct 2018; pediatric expansion Aug 2022).