Collection of an oseltamivir-resistant pandemic influenza A(H1N1) isolate in a 59-year-old man taking the drug prophylactically underscores the need to limit antiviral prophylaxis of pandemic flu, according to correspondence published online in the New England Journal of Medicine on Nov. 11.
Oseltamivir was prescribed to all household contacts of the man's 13-year-old son (the index patient), whose infection was confirmed by a reverse-transcriptase polymerase-chain reaction test. The patient's father, mother, and sisters (aged 15 and 18 years) were prescribed 75 mg oseltamivir once daily for 10 days as prophylaxis.
The patient's father was taking 5 mg prednisone daily and had chronic obstructive pulmonary disease. The father developed flulike symptoms approximately 24 hours after prophylaxis began.
On prophylaxis day 8, the father was seen by his physician for a persistent cough, and an H1N1-positive nasopharyngeal aspirate was collected. After an uneventful course of illness, a second sample was negative, the investigators wrote. The case was reported by Mariana Baz, M.Sc., of the Centre Hospitalier Universitaire de Québec, and colleagues (N. Engl. J. Med. 2009 Nov. 11 [Epub doi: 10.1056/NEJMC0910060]).
The sample from the father showed a neuraminidase mutation (H275Y) that has been associated with oseltamivir resistance in seasonal H1N1
"We hypothesize that the presence of subtherapeutic levels of oseltamivir at a time when viral replication had already begun was an important factor that led to the emergence of the resistant virus in the father of our index patient," they wrote.
The case supports the need to limit prophylactic treatment with oseltamivir in persons who have already been exposed to the H1N1 virus, the investigators noted. They also suggested that patients receiving once-daily antiviral prophylaxis convert to a twice-daily regimen as soon as they develop flulike symptoms.
A conflict of interest disclosure statement for the authors was unavailable at press time.