Wednesday, November 24, 2010

Study: Aerosolized Transmission Of Influenza



# 5084



Remarkably, even as we approach the end of the first year of the second decade of the 21st century, there remains a good deal of basic information about how influenza is transmitted that scientists haven’t completely nailed down.


The easy answer is via coughs and sneezes, but we actually need something a little more definitive than that.


The three commonly cited routes are large-droplets, aerosols, and  direct contact with secretions and fomites (inanimate objects contaminated with the influenza virus).


Most scientists concede that all three probably play a role in transmission, but how much each method contributes is far less certain.


Coughing and sneezing produces virus laden large-droplets that remain airborne for a very short period of time, and then settle to the ground (or other surfaces).


The `range’ of these droplets is assumed to be 6 to 10 feet, and has traditionally been considered the primary route of transmission.



Photo Credit PHIL (Public Health Image Library)


Virus particles may also end up deposited on fomiteslike phone receivers, computer keyboards, and shopping cart handles – and end up transferred to others that come in contact with them.


Coughs and sneezes – and certain medical procedures (like nebulizers) -  can also produce fine aerosolized particles that can conceivably remain airborne for extended periods and travel much further than large droplets.  


But how often, and under what conditions, aerosolized transmission of the influenza virus actually takes place remains a subject of considerable debate.   


Since those answers could affect infection control policies and recommendations, particularly in heath care settings, determining the actual mechanisms of influenza transmission is more than just an academic exercise.


While it doesn’t come close to definitively answering the question, today we’ve a new study that appears in IDSA’s journal Clinical Infectious Diseases that adds some more data to the mix.


A few excerpts from the Abstract (follow the link to read it in its entirety), followed by a little more discussion.


Clinical Infectious Diseases 2010;51:1176–1183

DOI: 10.1086/656743

Possible Role of Aerosol Transmission in a Hospital Outbreak of Influenza

Bonnie C. K. Wong,Nelson Lee,Yuguo Li,Paul K. S. Chan,Hong Qiu,Zhiwen Luo,Raymond W. M. Lai,Karry L. K. Ngai,David S. C. Hui,K. W. Choi,and Ignatius T. S. Yu

Background. We examined the role of aerosol transmission of influenza in an acute ward setting.

Methods. We investigated a seasonal influenza A outbreak that occurred in our general medical ward (with open bay ward layout) in 2008. Clinical and epidemiological information was collected in real time during the outbreak. Spatiotemporal analysis was performed to estimate the infection risk among patients. Airflow measurements were conducted, and concentrations of hypothetical virus‐laden aerosols at different ward locations were estimated using computational fluid dynamics modeling.

Results.Nine inpatients were infected with an identical strain of influenza A/H3N2 virus. With reference to the index patient’s location, the attack rate was 20.0% and 22.2% in the “same” and “adjacent” bays, respectively, but 0% in the “distant” bay (P=.04).



Conclusions. Our findings suggest a possible role of aerosol transmission of influenza in an acute ward setting. Source and engineering controls, such as avoiding aerosol generation and improving ventilation design, may warrant consideration to prevent nosocomial outbreaks.



Although the article isn’t open access, we can glean a few more details from the IDSA press release.


Infectious Diseases Society of America

Hong Kong hospital reports possible airborne influenza transmission

Direct contact and droplets are the primary ways influenza spreads. Under certain conditions, however, aerosol transmission is possible. In a study published in the current issue of Clinical Infectious Diseases, available online, the authors examined such an outbreak in their own hospital in Hong Kong.


On April 4, 2008, seven inpatients in the hospital's general medical ward developed fever and respiratory symptoms. Ultimately, nine inpatients exhibited influenza-like symptoms and tested positive for influenza A. The cause of the outbreak was believed to be an influenza patient who was admitted on March 27. He received a form of non-invasive ventilation on March 31, and was then moved to the intensive care unit after 16 hours. During that time, he was located right beside the outflow jet of an air purifier, which created an unopposed air current across the ward.


"We showed that infectious aerosols generated by a respiratory device applied to an influenza patient might have been blown across the hospital ward by an imbalanced indoor airflow, causing a major nosocomial outbreak," said study author Nelson Lee, MD, of the Chinese University of Hong Kong. "The spatial distribution of affected patients was highly consistent with an aerosol mode of transmission, as opposed to that expected from droplet transmission.


"Suitable personal protective equipment, including the use of N95 respirators, will need to be considered when aerosol-generating procedures are performed on influenza patients," Dr. Lee added. "Avoiding such procedures in open wards and improving ventilation design in health care facilities may also help to reduce the risk of nosocomial transmission of influenza."



In this case, it took the combination of an influenza patient receiving a ventilation treatment in an open ward next to another air current producing device to lead to the apparent aerosolized spread of the virus.


A fairly complex (but not altogether unusual) set of circumstances found in this particular emergency department.  


Under what other conditions influenza (and other respiratory viruses) might spread via aerosolized particles remains an open question.  


A few blogs (and conflicting studies) on this subject include:


Ferreting Out The Transmissibility Of Aerosolized H5N1
Study: H5N1 - A Very Persistent Virus
Good To The Last Droplet
Referral: Influenza and airborne transmission

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