Influenza is thought to be communicated from person to person by multiple pathways; however, the relative importance of different routes of influenza transmission is unclear. To better understand the potential for the airborne spread of influenza, William Lindsley, of the Allergy and Clinical Immunology Branch in the Health Effects Laboratory Division at the National Institute for Occupational Safety and Health, and colleagues measured the amount and size of aerosol particles containing influenza virus that were produced by coughing.
Subjects were recruited from patients presenting at a student health clinic with influenza-like symptoms. Nasopharyngeal swabs were collected from the volunteers and they were asked to cough three times into a spirometer. After each cough, the cough-generated aerosol was collected using a NIOSH two-stage bioaerosol cyclone sampler or an SKC BioSampler. The amount of influenza viral RNA contained in the samplers was analyzed using quantitative real-time reverse-transcription PCR (qPCR) targeting the matrix gene M1.
For half of the subjects, viral plaque assays were performed on the nasopharyngeal swabs and cough aerosol samples to determine if viable virus was present. Fifty-eight subjects were tested, of whom 47 were positive for influenza virus by qPCR. Influenza viral RNA was detected in coughs from 38 of these subjects (81 percent). Thirty-five percent of the influenza RNA was contained in particles >4 µm in aerodynamic diameter, while 23 percent was in particles 1 to 4 µm and 42 percent in particles <1 µm. Viable influenza virus was detected in the cough aerosols from 2 of 21 subjects with influenza.
The researchers say that these results show that coughing by influenza patients emits aerosol particles containing influenza virus and that much of the viral RNA is contained within particles in the respirable size range. The results support the idea that the airborne route may be a pathway for influenza transmission, especially in the immediate vicinity of an influenza patient. The researchers add that further research is needed on the viability of airborne influenza viruses and the risk of transmission.
This work was supported by internal funds from the National Institute for Occupational Safety and Health (NIOSH) and by an interagency agreement with the US Environmental Protection Agency (EPA).
Reference: Lindsley WG, Blachere FM, Thewlis RE, Vishnu A, Davis KA, Cao G, Palmer JE, Clark KE, Fisher MA, Khakoo R, Beezhold DH. Measurements of Airborne Influenza Virus in Aerosol Particles from Human Coughs. PLoS ONE 5(11): e15100. doi:10.1371/journal.pone.0015100