Influenza Virus Present in Fine Particles Collected During Breathing

Emma Hitt, PhD

March 18, 2008

March 18, 2008 (Atlanta) — Influenza virus is detectable in fine particles generated during normal breathing, which raises the question of whether the virus can be transmitted through the airborne route, according to new research.

Donald Milton, MD, DrPH, from the University of Massachusetts, Lowell, and colleagues reported their findings here at the 2008 International Conference on Emerging Infectious Diseases.

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. Notably, the current Department of Health and Human Services 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 N95 respirators (which do protect against airborne transmission) only for certain aerosol-generating medical procedures, such as endotracheal intubation and nebulizer treatment.

This study sought to characterize particle and influenza virus concentrations during exhalation from infected patients. Patients with influenza-like illness requesting treatment at clinics in Hong Kong, China, were tested for influenza transmission by rapid testing as well as quantitative polymerase chain reaction (PCR) testing of nasal swabs. Of 68 patients, 13 tested positive for influenza through rapid testing. Of those, 12 patients completed an exhaled breath test (5 had influenza A and 7 had influenza B).

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 3 minutes. Breath particles were collected on Teflon filters for 15 minutes and analyzed with quantitative PCR.

Influenza virus was obtained in the exhaled breath of 4 (25%) of the 12 participants, 3 of whom had influenza A and 1 who had influenza B. Exhaled breath virus concentrations ranged between fewer than 48 and 300 virus copies per sample, indicating a rate of between fewer than 3.2 and 20 virus particles generated per minute.

Two of the 4 patients exhaled more than 500 particles per liter of air, which has been recommended as a threshold for high particle producers.

"These findings add to data suggesting an important role for airborne transmission of influenza," Dr. Milton told Medscape Infectious Diseases. "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," he said. 

According to Dr. Milton, it is possible that viruses detected with quantitative PCR in fine-particle samples are not infectious. Future studies will include culturing of samples for infectious virus to better address this issue, he said.

The issue remains controversial, however. "Until we see whether infection actually occurs at long distances — longer than would be required for droplet transmission — one cannot glean from these findings that they are clinically relevant," said Camille Lemieux, MD, from the Toronto General Hospital, Ontario, Canada. "They suggest only that virus is detected in smaller respiratory particles," she added.

In a January 2007 letter to Emerging Infectious Diseases, Dr. Lemieux and colleagues noted that "the only reasonable conclusion that can be drawn at this time is that aerosol transmission does not play a major role in natural influenza epidemiology. Whether aerosols play any role in the transmission of influenza is a question demanding an answer; it is clear that we do not yet have that answer."

According to Dr. Lemieux, this is a very important issue, and there are still gaps in knowledge that are not addressed by the current study.

The study was funded by noncommercial sources, including grants from the Centers for Disease Control and Prevention and the US Federal Aviation Administration. The authors and independent commentators have disclosed no relevant financial relationships.

International Conference on Emerging Infectious Diseases 2008: Board 176. Presented March 18, 2008.

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