Pre-symptomatic healthcare worker transmission of pandemic (H1N1) influenza in acute care settings

Nosocomial influenza transmission is a concern in healthcare settings. In 2009, Los Angeles County Department of Public Health (DPH) investigated two outbreaks of pandemic (H1N1) influenza in acute care hospitals where transmission occurred during the pre-symptomatic infectious period from healthcare workers (HCW) to high-risk patients prior to release of the vaccine. Outbreak 1 occurred in July in the hematology-oncology ward of Facility A. The facility reported two nosocomial influenza cases diagnosed with pandemic influenza via rRT-PCR with onset five days apart. DPH investigation revealed HCW 1, prior to developing influenza-like illness, provided care to Case 1 for several days; Case 1 developed symptoms the same day as HCW 1. Indirect contact between Case 1 and Case 2 occurred through the parent of Case 2. HCW 1 was not tested for influenza. Both cases subsequently expired. Outbreak 2 occurred in July in the neonatal intensive care unit of Facility B. Three infants with influenza A (two confirmed as pandemic) were identified within a 24 hour period. DPH investigation revealed four ill HCWs. HCW 1 cared for Case 1 and Case 2 for two days prior to onset of illness. Three other HCWs who cared for Cases 2 and 3 became symptomatic 1-2 days after HCW 1. HCWs were not tested for influenza, and none worked while symptomatic. All infants recovered uneventfully. Both outbreaks demonstrate the possible transmission of influenza from pre-symptomatic HCWs and highlight that enhanced personal hygiene is critical for HCWs in high-risk patient settings, especially in the absence of an effective vaccine.