Viable influenza A virus in airborne particles expelled during coughs versus exhalations

Abstract

Background: To prepare for a possible influenza pandemic, a better understanding of the potential for the airborne transmission of influenza from person to person is needed.

Objectives: The objective of this study was to directly compare the generation of aerosol particles containing viable influenza virus during coughs and exhalations.

Methods: Sixty-one adult volunteer outpatients with influenza-like symptoms were asked to cough and exhale three times into a spirometer. Aerosol particles produced during coughing and exhalation were collected into liquid media using aerosol samplers. The samples were tested for the presence of viable influenza virus using a viral replication assay (VRA).

Results: Fifty-three test subjects tested positive for influenza A virus. Of these, 28 (53%) produced aerosol particles containing viable influenza A virus during coughing, and 22 (42%) produced aerosols with viable virus during exhalation. Thirteen subjects had both cough aerosol and exhalation aerosol samples that contained viable virus, 15 had positive cough aerosol samples but negative exhalation samples, and 9 had positive exhalation samples but negative cough samples.

Conclusions: Viable influenza A virus was detected more often in cough aerosol particles than in exhalation aerosol particles, but the difference was not large. Because individuals breathe much more often than they cough, these results suggest that breathing may generate more airborne infectious material than coughing over time. However, both respiratory activities could be important in airborne influenza transmission. Our results are also consistent with the theory that much of the aerosol containing viable influenza originates deep in the lungs.

Keywords: Aerosols; air microbiology; airborne transmission; cough; infectious disease transmission; influenza.

Figure 1

Collection system for airborne particles produced by subjects during coughing and exhalation. Before each respiratory activity, the piston spirometer was purged and partially filled with 4 liters of dry filtered air. The subject then sealed their mouth around the mouthpiece and coughed or exhaled as instructed. The cough or exhalation traveled through the ultrasonic spirometer, which measured the volume and flow rate, and then into the piston spirometer. When the subject was finished, the valve was closed and the SKC BioSampler was used to collect the aerosol particles produced by the subject.

Figure 2

Electrophoretic gel used to determine the presence or absence of a 101‐base pair PCR product corresponding to the influenza A M1 matrix gene. The PCR products for the NOP swabs, cough aerosols, and exhalation aerosols for three test subjects are shown. (+) indicates the sample is positive for influenza A. (−) indicates the sample is negative. The PCR products for the cough and exhalation samples for subject FC178 were confirmed to be from the influenza A M1 matrix gene by DNA sequence analysis. The negative control contained all PCR reagents, primers, and probe but no template. The positive control contained 10⁴ M1 copies and was run in parallel with the experimental samples.