Photo Credit PHIL (Public Health Image Library)
Although coughs and sneezes are universally blamed for spreading the flu, surprisingly - exactly how, and how far, and for how long – the virus spreads in the air has long been the subject of debate.
The mantra for many years has been that large-droplets are probably the primary mode of transmission, as described in this CDC report from 2009:
Influenza viruses are spread from person to person primarily through large-particle respiratory droplet transmission (e.g., when an infected person coughs or sneezes near a susceptible person). Transmission via large-particle droplets requires close contact between source and recipient persons, because droplets do not remain suspended in the air and generally travel only a short distance (less than or equal to 1 meter) through the air.
Contact with respiratory-droplet contaminated surfaces is another possible source of transmission.
Airborne transmission (via small-particle residue [less than or equal to 5µm] of evaporated droplets that might remain suspended in the air for long periods of time) also is thought to be possible, although data supporting airborne transmission are limited.
Recommendations for infection control – particularly in health care settings – have generally focused on surgical masks to prevent large droplet transmission, and handwashing to stop fomite transmission of the virus.
In recent years, however, we’ve seen research suggesting that aerosolized particles may play a much bigger role in the transmission of influenza than previously suspected.
Last March, in With Influenza Virus Particles, Size Matters we looked at research from the University of Maryland that found Large droplets - long believed the the primary delivery vessel of influenza viruses - actually contained far fewer viral copies than did fine (≤5 µm) aerosol particles.
This study not only found aerosolized transmission was probably more common than previously believed, they found that surgical masks, when worn by the person who is infected, do a credible job of protecting others from the virus.
Another study conducted at Wake Forest School of Medicine, which we looked at in Influenza Transmission, PPEs & `Super Emitters’, questioned the current wisdom on how flu is transmitted. Some of its highlights included:
1) Researchers found that patients with influenza can emit small, influenza virus-containing particles into the surrounding air during routine patient care, potentially exposing health care providers to influenza virus up to 6 feet away from infected patients.
2) Five patients (19 percent) in study were "super-emitters" who emitted up to 32 times more virus than others. Patients who emit a higher concentration of influenza virus also reported greater severity of illness.
3) The findings suggest that more research on how influenza is transmitted is needed and that current influenza infection control recommendations for health providers may need to be reevaluated.
In November of 2010, in Study: Aerosolized Transmission Of Influenza, we saw a report on the nosocomial spread of influenza believed to be caused by infectious aerosols spread by an imbalanced indoor airflow.
Another study (see Study: Aerosolized Influenza And PPEs) from March of 2012, looked at the effectiveness of various types of PPEs.
All of which serves as prelude to a new study, published yesterday in Nature Communications by a team of international researchers (including such familiar names as Malik Peiris, Benjamin J. Cowling, and Timothy M. Uyeki).
We’ve got the abstract, and as an added bonus, an audio interview with Dr. Cowling courtesy of Radio Australia.
Not only did these researchers find that roughly half of all influenza transmissions are caused by aerosols, they found that infections from smaller aerosol particles were more likely to make you sicker.
First the link to the study (the bulk of which is behind a pay wall), and then a link to the Interview.
Benjamin J. Cowling,Dennis K. M. Ip,Vicky J. Fang, Piyarat Suntarattiwong,Sonja J. Olsen,Jens Levy, Timothy M. Uyeki,Gabriel M. Leung,J. S. Malik Peiris,Tawee Chotpitayasunondh, Hiroshi Nishiura & James Mark Simmerman
Influenza A viruses are believed to spread between humans through contact, large respiratory droplets and small particle droplet nuclei (aerosols), but the relative importance of each of these modes of transmission is unclear. Volunteer studies suggest that infections via aerosol transmission may have a higher risk of febrile illness.
We find that aerosol transmission accounts for approximately half of all transmission events. This implies that measures to reduce transmission by contact or large droplets may not be sufficient to control influenza A virus transmission in households.
The radio interview (click on the image or link) contains extended comments not in the transcription. Well worth the 4 minutes to listen to in its entirety.
Updated 5 June 2013, 15:05 AEST
A team of international researchers say they've made an important discovery about the transmission of the flu virus. They believe their findings should prompt a change in the way Australians tackle the spread of the flu.
Presenter: Samantha Donovan
Speaker: Ben Cowling, School of Public Health at the University of Hong Kong
Among the topics Dr. Cowling discusses are the potential for environmental controls (temperature & humidity) to play a role in reducing the spread of influenza. This is a topic we looked at last fall in:
Despite decades of research, our knowledge of how influenza spreads, and what barriers work well to protect HCWs, remains limited. A few other relevant studies we’ve looked in the past include: