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Is Airborne Spread of Influenza Possible?

By Amesh A. Adalja, MD, January 22, 2009

A new study1 from researchers at West Virginia University demonstrates that the influenza virus is able to remain suspended in the air, thus enabling airborne transmission. This finding suggests the possible need for more stringent infection control measures to forestall spread of seasonal influenza within hospitals. 

Emergency Department Air Sampled for Virus

For this study, which took place during the 2008 flu season, Blachere and colleagues sampled air within an emergency department using a device with different caliber inlets and outlets to collect different sized particles. Both stationary and movable samplers were employed on 6 different occasions in a variety of locations, including the waiting room and examination rooms. Physicians wore personal samplers. Polymerase chain reaction (PCR) amplification was then performed to determine whether the genetic signature of influenza could be found in the samples.1

Influenza Virus Found in Aerosol Samples

On 3 separate days, influenza A virus was found, with more than 50% of the isolates derived from the stage of the air sampler specific for aerosolized particles (less than 4 micrometer diameter). Samplers in the waiting room, reception room, and those worn by physicians were all positive.1

Are More Stringent Infection Control Measures Needed for Influenza A?

While the mode of transmission of influenza is complex and poorly understood, it is a critically important issue, especially in the healthcare setting. Three mechanisms of transmission have been suggested: direct contact, large respiratory droplets that fall quickly to the ground within a few feet, and small respiratory aerosols that remain suspended in the air for considerable time and travel considerable distance. However, there is limited and inconclusive evidence for each mechanism. While all three may be at play, the predominant mode of transmission is still not known. Current recommendations for managing patients with seasonal influenza call for droplet precautions—chiefly the use of simple surgical masks—for prevention of nosocomial spread of the virus.

The results of Blachere’s study call into question current recommendations for utilizing only droplet precautions when caring for patients with influenza. Given the presence of aerosolized virus, hospital air itself may be contaminated and act as a vector for spread of the influenza virus, just as hospital air can spread tuberculosis.

Infection control measures for human cases of H5N1 influenza that call for use of airborne precautions reflect this danger. While those precautions likely stem from the high pathogenicity of the virus rather than from any evidence of airborne spread,2 it is conceivable that this paradigm may need to be adopted as a matter of course to better protect patients and healthcare workers from contracting seasonal influenza virus in healthcare facilities.

Nosocomial spread of influenza, which is responsible for 28% of influenza cases in some hospitals,3 is very costly, with estimates reaching up to $3800 per case.4 Coupled with the alarmingly high rates of influenza A H1N1 resistance to oseltamavir and H3N2 resistance to adamantanes,5 infection control should be given higher priority in influenza planning. The best strategy for disrupting the spread of influenza may require rapid testing of all suspected cases and initiation of infection control measures similar to those used with persons suspected to have tuberculosis, which may entail the use of N-95 respirators and negative pressure rooms. Further research to verify the airborne transmissibility of influenza A is needed.


  1. Blachere FM, Lindsley WG, Pearce TA, et al. Measurement of Airborne Influenza Virus in a Hospital Emergency Department. Clin Infect Dis. 2009; [Epub ahead of print]. Available at: Accessed January 19, 2009.

  2. Interim Recommendations for Infection Control in Health-Care Facilities Caring for Patients with Known or Suspected Avian Influenza. CDC. May 21, 2004. Available at: Accessed January 19, 2009.

  3. Slinger R, Dennis P. Nosocomial Influenza at a Canadian Pediatric Hospital from 1995 to 1999: Opportunities for Prevention.  Infect Control Hosp Epidemiol.2002;23:627–629.

  4. Sartor C, Zandotti S, Romain F, et al. Disruption of Services in an Internal Medicine Unit Due to a Nosocomial Influenza Outbreak. Infect Control Hosp Epidem. 2002. 23:615-19.

  5. Hauge SH, Dudman S, Borgen K, Lackenby A, Hungnes O. Oseltamivir-resistant influenza viruses A (H1N1), Norway, 2007–08. Emerg Infect Dis. 2009 Feb; [Epub ahead of print]. Available at: Accessed January 19, 2009.