What is the recommended post-exposure prophylaxis for an individual exposed to influenza, who is not vaccinated and has no contraindications to medication?

Post-Exposure Prophylaxis for Influenza

For an unvaccinated individual exposed to influenza with no medication contraindications, initiate oseltamivir 75 mg once daily for 10 days, starting as soon as possible and ideally within 48 hours of exposure. 1, 2

Timing is Critical

• Prophylaxis must be initiated within 48 hours of exposure to be optimally effective. 1, 3 The protective benefit is greatest when started immediately after exposure is identified. 1
• If more than 48 hours has elapsed since exposure, do not initiate prophylaxis; instead, educate the patient to start full treatment dosing (75 mg twice daily) immediately if symptoms develop. 1

Standard Dosing Regimen

• Adults and adolescents ≥13 years: oseltamivir 75 mg once daily for 10 days after last known exposure. 4, 1, 2
• The duration of protection lasts only as long as the medication is continued—susceptibility to influenza returns when prophylaxis is discontinued. 4, 2

Alternative Agent

• Zanamivir 10 mg (two 5-mg inhalations) once daily for 7-10 days is an alternative for patients ≥5 years who cannot tolerate oseltamivir or when oseltamivir resistance is suspected. 1, 3
• Baloxavir is a newer single-dose alternative for patients ≥12 years, demonstrating a 1% infection rate versus 13% with placebo when given within 48 hours of exposure. 3

Concurrent Vaccination

• Oseltamivir prophylaxis does not interfere with inactivated influenza vaccine (IIV) and both can be administered simultaneously. 1 This is particularly important for unvaccinated individuals, as vaccination remains the primary prevention strategy. 4, 3
• However, if live attenuated influenza vaccine (LAIV) is used, oseltamivir will decrease its effectiveness, so timing considerations are needed. 1
• For high-risk individuals who just received vaccination, continue prophylaxis for 2 weeks until optimal immunity develops. 4

Important Clinical Considerations

• Oseltamivir should be taken with food to reduce gastrointestinal side effects, particularly nausea (occurs in ~10-15% of patients) and vomiting. 5, 6
• The most common adverse effects are nausea (12-15%) and vomiting (2.5-3%), but these are generally mild and transient. 6, 7
• Prophylaxis is not a substitute for annual influenza vaccination—it is an adjunct measure only. 4, 1, 3

When to Monitor for Treatment Failure

• If symptoms develop during prophylaxis, test for influenza and switch to treatment dosing (75 mg twice daily for 5 days). 1
• Consider using an antiviral with a different resistance profile if breakthrough infection occurs. 1

Common Pitfalls to Avoid

• Do not delay prophylaxis waiting for laboratory confirmation of the index case—start empirically if clinical suspicion is high and the exposure occurred within 48 hours. 4
• Do not use amantadine or rimantadine, as resistance rates exceed 99% among currently circulating influenza A viruses. 3
• Remember that prophylaxis only protects during the medication period—risk returns immediately after discontinuation. 4, 2