BACKGROUND:

Oseltamivir resistance among 2009 pandemic influenza A (H1N1) viruses (pH1N1) is rare. We investigated a cluster of oseltamivir-resistant pH1N1 infections in a hospital ward.

METHODS:

We reviewed patient records and infection control measures and interviewed health care personnel (HCP) and visitors. Oseltamivir-resistant pH1N1 infections were found with real-time reverse-transcription polymerase chain reaction and pyrosequencing for the H275Y neuraminidase (NA) mutation. We compared hemagglutinin (HA) sequences from clinical samples from the outbreak with those of other surveillance viruses.

RESULTS:

During the period 6-11 October 2009, 4 immunocompromised patients within a hematology-oncology ward exhibited symptoms of pH1N1 infection. The likely index patient became febrile 8 days after completing a course of oseltamivir; isolation was instituted 9 days after symptom onset. Three other case patients developed symptoms 1, 3, and 5 days after the index patient. Three case patients were located in adjacent rooms. HA and NA sequences from case patients were identical. Twelve HCP and 6 visitors reported influenza symptoms during the study period. No other pH1N1 isolates from the hospital or from throughout the state carried the H275Y mutation.

CONCLUSIONS:

Geographic proximity, temporal clustering, presence of H275Y mutation, and viral sequence homology confirmed nosocomial transmission of oseltamivir-resistant pH1N1. Diagnostic vigilance and prompt isolation may prevent nosocomial transmission of influenza.