Influenza Virus Detected in Fine Particles Collected During Breathing

Influenza virus is detectable in fine particles generated during normal breathing, raising the question of whether the virus can be transmitted through the airborne route.

Researchers from the University of Massachusetts (Lowell, USA) tested patients with influenza-like illness who had requested treatment at clinics in Hong Kong (SAR, China) for influenza transmission by rapid testing as well as quantitative polymerase chain reaction (PCR) testing of nasal swabs. For the breath test, patients breathed high-efficiency particulate filtered air into 40-cm long tubes containing an optical particle counter. Particles between 0.3 and 5 µm in size were counted for three minutes. Breath particles were also collected on Teflon filters for 15 minutes and analyzed with PCR.

The results showed that of 68 patients, 13 tested positive for influenza through rapid testing. Of those, five patients who had influenza A and seven who had influenza B completed an exhaled breath test. Influenza virus was obtained in the exhaled breath of four (25%) of the 12 participants, three of whom had influenza A and one who had influenza B. Exhaled breath virus concentrations ranged between fewer than 48 and up to 300 virus copies per sample, indicating a rate of between fewer than 3.2 and 20 virus particles generated per minute. Two of the four patients exhaled more than 500 particles per liter of air, which has been recommended as a threshold for high particle producers. The study findings were reported at the 2008 International Conference on Emerging Infectious Diseases, held during March 2008 in Atlanta (GA, USA).

"These findings add to data suggesting an important role for airborne transmission of influenza,” said lead author Donald Milton, M.D., Dr.PH. "Now that we have shown that we can detect virus in the aerosol, the next step will be to test the effectiveness of face masks for containing the virus aerosols emitted by patients.”

At present, influenza is thought to be transmitted through large droplets and by contact, but the contribution of airborne transmission to the spread of influenza is uncertain. The current U.S. Department of Health and Human Services (Washington, D.C., USA) pandemic influenza plan recommends the use of surgical masks--which do not protect against airborne transmission--when treating patients with influenza and reserves the use of protective N95 respirators only for certain aerosol-generating medical procedures, such as endotracheal intubations and nebulizer treatment.


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