Abstract
S-29A8-3 Background/Aims: The scientific literature on influenza transmission is lacking in data on the particle size, infectiousness, and quantity of viral particles released by infected persons. These data are necessary to understand the transmission pathways of influenza and are essential for assessing risk, providing interventions, and minimizing public fear. Methods: We characterized exhaled breath and cough aerosols from influenza patients using molecular and viral culture methods during seasonal influenza in Hong Kong 2007, Massachusetts 2008, and 2009. Using filters or a novel impaction system to collect exhaled virus aerosols, we estimated virus generation rates during tidal breathing and coughing. We measured particle concentrations and size distributions with an optical particle counter. During the third season we measured the effect of surgical masks on virus concentrations in fine (0.05–<5 um) and coarse (≥5 um) particles as well as virus culturability from fine particles. Results: We recruited 77 subjects with laboratory-confirmed influenza during the 3 seasons. We measured influenza RNA in 13%–33% of tidal breath samples and in 66% of cough samples. Over 87% of exhaled particles collected were less than 1 μm. Surgical masks reduced viral RNA copies in the coarse fraction from 1000 per 30 minutes to 2.6 per 30 minutes and in the fine-particle fraction from 5600 per 30 minutes to 1200 per 30 minutes. Overall, masks produced a 5.2-fold reduction in viral aerosol shedding. Samples from the 2 subjects with the highest influenza RNA copy numbers were culture positive. Conclusion: Results from these studies show that influenza virus is present in fine particles generated during tidal breathing and coughing. These results and the presence of culturable virus in fine-particle aerosols support the hypothesis that the airborne route may play a role in influenza transmission. Data from 1 study suggest that surgical masks worn by patients can moderately reduce generation of virus containing aerosols and supports Center for Disease Control recommendations.
Highlights
Transmission of influenza virus between humans may occur by three routes: (1) direct or indirect contact between an infected and a susceptible person, usually resulting in contamination of a susceptible person’s hands followed by hand to respiratory mucosa contact; (2) large droplet spray wherein droplets of respiratory fluid greater than approximately 100 mm in diameter are expelled with sufficient momentum to deliver a direct hit on the respiratory mucosa; and (3) aerosols generated by release of smaller, viruscontaining droplets, as may occur during tidal breathing and coughing [1,2], that rapidly evaporate into residual particles,which are inhaled and deposited in the respiratory tract [3,4,5,6]
We sought to determine the total number of viral RNA copies present in exhaled breath and cough aerosols, whether the RNA copies in fine particle aerosols represent infectious virus, and whether surgical facemasks reduce the amount of virus shed into aerosols by people infected with seasonal influenza viruses
We found that total viral copies detected by molecular methods were 8.8 times more numerous in fine (#5 mm) than in coarse (.5 mm) aerosol particles and that the fine particles from cases with the highest total number of viral RNA copies contained infectious virus
Summary
Transmission of influenza virus between humans may occur by three routes: (1) direct or indirect contact between an infected and a susceptible person, usually resulting in contamination of a susceptible person’s hands followed by hand to respiratory mucosa contact; (2) large droplet spray wherein droplets of respiratory fluid greater than approximately 100 mm in diameter are expelled with sufficient momentum to deliver a direct hit on the respiratory mucosa; and (3) aerosols generated by release of smaller, viruscontaining droplets, as may occur during tidal breathing and coughing [1,2], that rapidly evaporate into residual particles (droplet nuclei),which are inhaled and deposited in the respiratory tract [3,4,5,6]. An experimental study using intranasal inoculation to infect experimental donors [11] will need to show that the donors and naturally infected persons shed similar virus aerosols with regard to quantity, particle size distribution, and infectiousness, given that earlier experiments suggested that intranasal inoculation requires quantitatively larger doses and produces qualitatively milder illness than does inoculation via aerosol [12]
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