Flu Prophylaxis Medication
Neuraminidase inhibitors (NAIs), specifically oseltamivir, are the recommended agents for influenza chemoprophylaxis, with adamantanes (amantadine/rimantadine) no longer recommended due to high resistance rates. 1, 2
Primary Prophylaxis Agents
Oseltamivir (First-Line)
Oseltamivir is the only FDA-approved neuraminidase inhibitor for prophylaxis and should be used preferentially over adamantanes. 1, 3
Standard prophylaxis dosing:
- Adults and adolescents ≥13 years: 75 mg once daily 2, 3
- Pediatric patients (weight-based): 3
- ≤15 kg: 30 mg once daily
15-23 kg: 45 mg once daily
23-40 kg: 60 mg once daily
40 kg: 75 mg once daily
Zanamivir (Alternative)
While zanamivir has demonstrated similar efficacy to oseltamivir in preventing influenza (84% vs. 82% efficacy), only oseltamivir is FDA-approved for prophylaxis. 1 Zanamivir may be considered when oseltamivir resistance is suspected or confirmed. 4
Adamantanes (Not Recommended)
Amantadine and rimantadine are no longer recommended for prophylaxis due to high resistance rates (>95%) among circulating influenza A strains and complete lack of activity against influenza B. 1, 4 Historical guidelines from 1992-2004 recommended these agents when resistance was low (70-90% efficacy), but current resistance patterns have rendered them obsolete. 1
Clinical Indications for Prophylaxis
Pre-Exposure (Seasonal) Prophylaxis
Duration: Throughout influenza season or during peak community activity (up to 6 weeks, or up to 12 weeks in immunocompromised patients) 1, 2, 3
High-priority candidates: 1, 2
- Severely immunocompromised patients (hematopoietic stem cell transplant recipients within 6-12 months post-transplant, lung transplant recipients)
- Patients for whom vaccination is contraindicated, unavailable, or expected to have low effectiveness
- Unvaccinated high-risk patients when influenza activity is detected in the community
Post-Exposure Prophylaxis
Duration: 10 days following exposure 2, 3
Initiation timing: As soon as possible after exposure, ideally within 48 hours 2
- Asymptomatic persons at very high risk of complications after household exposure
- Unvaccinated household contacts of persons at very high risk (in conjunction with vaccination)
- Healthcare workers during institutional outbreaks, particularly unvaccinated staff caring for high-risk patients
A three-day regimen of oseltamivir demonstrated 93% protective efficacy in hospital ward settings, comparable to standard 7-10 day regimens, though this shorter duration is not yet guideline-recommended. 5
Institutional Outbreak Control
During confirmed or suspected influenza outbreaks in nursing homes or chronic care facilities: 1
- Administer prophylaxis to all residents regardless of vaccination status
- Continue for ≥2 weeks or until approximately 1 week after outbreak ends
- Offer to unvaccinated staff providing care to high-risk persons
- Consider for all employees regardless of vaccination status if outbreak caused by vaccine-mismatched strain
Bridge Prophylaxis After Late Vaccination
For high-risk patients vaccinated after influenza activity has begun: 1
- Administer prophylaxis from time of vaccination until immunity develops (approximately 2 weeks in adults)
- Children <9 years receiving vaccine for first time may require up to 6 weeks of prophylaxis (4 weeks after first dose plus 2 weeks after second dose)
Special Populations and Dosing Adjustments
Renal Impairment
Critical dose adjustments required for creatinine clearance ≤60 mL/min: 2, 3
| Creatinine Clearance | Prophylaxis Dose |
|---|---|
| >60-90 mL/min | 75 mg once daily |
| >30-60 mL/min | 30 mg once daily |
| >10-30 mL/min | 30 mg every other day |
| ESRD on hemodialysis | 30 mg immediately, then 30 mg after alternate hemodialysis cycles |
| ESRD on CAPD | 30 mg immediately, then 30 mg once weekly |
| ESRD not on dialysis | Not recommended |
Immunocompromised Patients
Extended prophylaxis duration (up to 12 weeks) may be appropriate for severely immunocompromised patients during community outbreaks. 2, 6, 3 These patients should be monitored closely, as they may require resistance testing if breakthrough infection occurs. 6
Pregnant Women
Oseltamivir is recommended for pregnant women requiring prophylaxis, as benefits outweigh risks during pregnancy. 2
Important Clinical Considerations
Efficacy Data
- Seasonal prophylaxis: 74-82% protective efficacy in healthy adults over 6 weeks 7
- Post-exposure prophylaxis: 58.5-89% efficacy when started within 48 hours of household exposure 2
- Institutional settings: 92% reduction in influenza illness among nursing home residents 1
Common Pitfalls to Avoid
Do not use prophylaxis as substitute for vaccination. Annual influenza vaccination remains the primary prevention strategy. 2 Prophylaxis is adjunctive, not replacement therapy.
Do not administer prophylaxis routinely or for widespread use outside institutional outbreaks. 1 Prophylaxis should be targeted to specific high-risk situations to minimize resistance development and cost.
Do not delay prophylaxis while awaiting laboratory confirmation during outbreaks. Empiric prophylaxis based on clinical suspicion and local influenza activity is appropriate. 2
Monitor for breakthrough infections. If symptomatic illness develops during prophylaxis, test for influenza and switch to treatment dosing (75 mg twice daily), preferably with an antiviral with different resistance profile if not contraindicated. 1
Adverse Effects
The most common adverse effects are gastrointestinal: 2, 8, 7
- Nausea: 12-15% (vs. 7% placebo)
- Vomiting: 2.5-2.7% (vs. 0.8% placebo)
- Taking oseltamivir with food reduces gastrointestinal side effects
Premature discontinuation rates are similar to placebo (3-4%). 7 No established link between oseltamivir and neuropsychiatric events has been confirmed. 2
Resistance Considerations
Oseltamivir resistance remains low (<5%) in current circulating strains in the United States. 4 However, resistance may be higher in children (up to 18% in some studies) and immunocompromised patients with prolonged viral shedding. 4, 6
Alternative to Prophylaxis
Clinicians can consider educating patients and arranging for early empiric initiation of antiviral treatment as an alternative to prophylaxis, particularly for post-exposure situations. 1, 2 This approach requires patient access to medication and ability to recognize symptoms promptly for treatment within 48 hours of onset.