Influenza in the Hospital: Droplet or Airborne?

Exposure to Influenza Virus Aerosols During Routine Patient Care

Bischoff WE, Swett K, Leng I, Peters TR
J Infect Dis. 2013;207:1037-1046

Droplet or Aerosol?

Influenza is thought to be transmitted primarily by droplets (particles ≥ 5 μm, and settling within 3-6 feet); however, the airborne route (particles < 5 μm remaining suspended in air) has been implicated in special situations, such as aerosol-generating procedures. Discussion has been ongoing between the US Occupational Safety and Health Administration (OSHA) and healthcare epidemiologists on the relative role of the airborne route, especially during the 2009-2010 influenza season with the emergence of the pandemic H1N1 strain.

Bischoff and colleagues attempted to answer the question as to whether patients can produce influenza-containing aerosols under typical hospital circumstances.

Study Summary

During the 2010-2011 influenza season, a convenience sample of patients aged 2 years or older in a tertiary care teaching hospital were screened for the presence of influenza-like illness as defined by Centers for Disease Control and Prevention criteria. Real-time polymerase chain reaction (PCR) was performed on nasopharyngeal samples from patients and from air samples of patient rooms (with 6 air changes/hour at standard temperature and humidity).

Of the 94 patients with influenza-like illness, half were men or white, 60% were aged 18 years or older, and less than one third had received the seasonal influenza vaccine. A total of 31 patients had influenza A, 30 had influenza B, and the remaining 33 patients had neither type of influenza detected by PCR.

Aerosolized influenza virus was detected in 26 of 61 (43%) patients with influenza, of whom 5 ("superemitters") generated aerosols with 32 times more virus than the others. Viral concentrations measured at 6 feet were above the 50% infectious dose in 1%-10% of patients with influenza-like illness, depending on the threshold used, with 89% being small (< 4.7 μm) particles. Emitters had higher viral loads on PCR, but there were few clinical differences. Less than one half of patients with PCR-confirmed influenza received antiviral therapy.