“We are no more in the aerosol camp than the contact camp” conclude the authors. And this seems to be how it is in terms of influenza transmission routes – you’re either in one camp or the other. This 2010 PLoS Computational Biology paper is hardly hot off the press, but it is important and it does, to an extent, put the question of which camp you are in for influenza transmission to bed: you need to pitch your tent in different camps depending on the circumstances.
The paper describes a model to compare the various transmission routes for influenza, principally airborne, droplet and contact. The study evaluates four transmission routes: ‘respirable particles’ (<10 µm), ‘inspirable particles’ (>10 µm, <100 µm), ‘direct droplet spray’ (>100 µm) and ‘contact’. The model tests 10,000 scenarios, considering possible variation in virus properties, host susceptibility and environmental factors (such as the number of influenza shedders).
The key finding is that contact transmission had the highest average basic reproduction number (R0) (1.7) followed by droplet (0.27), respirable (0.05) and inspirable (0.006) particles (Figure). However, that is only part of the story. Of the 10,000 scenarios evaluated, contact only was associated with high transmission in 3,069, all four routes in 342 and none in 4,765. In high host density settings, all routes were more frequently important. Conversely, when self-inoculation was more common (i.e. when simulated individuals touched their simulated nose, eyes and mouths more frequently), contact transmission was more important.
Figure: Basic reproduction number (R0) of four influenza transmission routes, ‘respirable particles’ (<10 µm), ‘inspirable particles’ (>10 µm, <100 µm), ‘direct droplet spray’ (>100 µm) and ‘contact’.
The findings are interesting and probably very important. It’s a shame they were not able to evaluate the relative importance of contact transmission involving contaminated surfaces compared with contact transmission that occurs independent of surface transmission (this has been evaluated elsewhere). Also, I remain suspicious of modeling in general. If simplifying assumptions are too simplistic (which is often the case), the model spits out garbage, which is worse than useless. Put another way, Bertha can produce anything if she’s given the right inputs! Plus, it’s difficult to know how applicable these findings are to other respiratory viruses.
Still, the paper does shed light on the relative importance of influenza transmission routes. Which is most important? Well, that depends on the context. If you’re in a small room, airborne and droplet transmission is key. If you’re admitted to a room following the discharge of a patient with influenza, then contact transmission is key. Hence, we need to be flexible when considering influenza transmission routes and ‘contextualize’ our interventions accordingly.
Image: Sanofi Pasteur.
Reflections on Infection Prevention and Control 
Jon,
While this is an important topic, it’s so difficult to nail this down. Humidity, temperature, differences in attachment and/or survival for the numerous strains of influenza, and the list goes on and on regarding transmission routes. I do agree though with respect to the context of the transmission and/or exposure. These data could play a role in environmental decontamination practices and/or patient triage during influenza (and other respiratory viruses) season
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Totally agree Rodney. There are many, many variables here that will influence the predominant transmission route. For example, this model assumed on fixed rate of decay in terms of environmental survival, whereas influenza survival time on dry surfaces ranges from hours to weeks, depending on strain and experimental conditions. If you have a “survivor”, then contact transmission will be relatively more important than if you have a “weakling” that dies in hours on surfaces!
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One camp or the other is always a mistake. A sound layered approach to infection control is what makes sense to any one who is thinking. ASHRAE notes that any particle .3 mircons in size or smaller can remain airborne indefinitely. When you think of bacteria and viruses as particles you can clearly see they have the ability to move through the air.
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This is a timely post Jon, given some of the ongoing debates about communicability of H7N9 or MERS. I am a believer that if something has the potential( even very little) to spread by droplet then regardless that contact spread is more highly probable as a spread method. I would still recommend droplet. This is the same argument I would use to justify airborne precautions for MERS(resources permitting of course). I love the study methodology and thanks for sharing Jon. In the mean while I have to admit I belong to Team Droplet and I approve this message.
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We need to contextualise our understanding of transmission routes. Airborne, droplet and contact are all plausible for influenza – setting will determine which is most important. So, we need a foot in each camp!
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