Friday, February 1, 2013

Living and breathing = aerosol-generating procedure

Eli, who is busy this week attending ScienceOnline2013, pointed me to this newly published study in Journal of Infectious Diseases on influenza transmission. Werner Bischoff and colleagues at Wake Forest measured influenza virus RNA concentrations in air samples taken between 1-6 feet from influenza-infected patients’ heads during routine care. Among the 61 influenza patients they analyzed, 26 released measurable influenza RNA into room air, and 5 did so in very high concentrations. The figure below, from the paper, shows that small particle aerosols containing influenza RNA could be detected 6 feet from the patient’s head (for 9 patients, at levels exceeding their low estimate for a 50% human infectious dose).

What does this mean? Well, this report confirms that some of our influenza patients (those we’ve previously termed “superspreaders”) expel airborne virus in small particles (capable of long-distance spread) even when they aren’t undergoing an “aerosol-generating procedure”. Life with influenza, for them, is an aerosol-generating procedure.

How to translate this into reduced transmission in healthcare settings is tricky, and Caroline Breese Hall’s commentary is worth reading in this regard. A couple things to keep in mind—these investigators measured RNA, not viable virus. And even if we assume transmissibility from the RNA numbers, only a small number of the 61 influenza patients were high-concentration small-particle aerosol emitters. These data alone don't support making sweeping changes in practice for all patients with influenza-like illness (e.g. N95 masks, negative pressure rooms). The challenge is to learn how to identify potential “high risk emitters” early, or to identify specific settings when practice change is needed (based upon dynamics of the community outbreak, etc.).

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