What is the best approach to determine whether antiviral post-exposure prophylaxis for influenza is appropriate in an elderly immunocompromised patient who has been vaccinated?  

This is purely a matter of clinical judgement. The studies showing the benefits of post-exposure prophylaxis did not use any laboratory data, but it is believed that starting prophylaxis within 48h of exposure is important. The risk of severe complications or death in the hypothetical is unknown, bu...