A visual representation of how to prevent transmission of SARS-CoV-2 struck a chord with many in 2020. I won’t rehash all that has already been written about the Swiss cheese infographic but this is a small timeline, some links and some of the thinking that went on. Much of the detail is already on the New York Times piece along with a very sun-gazing photo of yours truly if you need content for the office dartboard.
Professor James T Reason proposed a layered approach to reducing the risk of accidents due to human error.
I really liked this approach for communicating risk reduction. Adapting it creates a simple way to get across that no single intervention (layer) is perfect, Each layer has its failings (holes) that can come and go and be made worse. Using lots of layers provides a better chance of preventing transmission of SARS-CoV-2, the virus that causes COVID-19.
By the way, in Reason’s 1990 publication, this model wasn’t associated with cheese.
No Swiss cheese here – but a hole (limited window of accident opportunity) is apparent in an inadequate defence. Reason intended the order of these layers to be relevant, but for the COVID-19 Swiss cheese defence model version I put together, I moved away from the order of layers being important in favour of the package of layers (risk-reducing interventions) being key to preventing transmission SARS-CoV-2.
This is Figure 1 copied from J Reason, “The contribution of latent human failures to the breakdown of complex systems”, Phil. Trans. R. Soc. Lond. B. 327, 475-484 (1990) 
Swiss cheese model (SCM)
In 2000, Prof Reason nicely illustrated his concept for a Swiss cheese model to describe a system approach to overcome innate human fallibilities.
Figure copied from James Reason’s “Human error: models and management”, BMJ 2000; 320: 768-70. 
Where do these holes come from? Let’s look at masks. We know they are good for preventing the wearer from spreading their virus-laden aerosols to others and into the air. They also serve some role in preventing a wearer from inhaling those aerosols. So what causes holes in this imperfect layer of risk reduction? Here are some ideas…
- The mask is not worn properly (under the nose, on the chin, too loose)
- Not disposing of the mask properly, putting others at risk of handling it
- Not sanitising your hands after handling the mask
- Fiddling with the outside of the mask while wearing it without sanitising your hands before touching your face or someone else
- Not cleaning your mask properly or often enough
- Sharing masks
- Not using the right type of fabric or enough layers for your mask
- Overconfidence that wearing a mask will be highly protective and thus engaging in risky behaviours as a result of that confidence
And those are just some of the holes in one layer!
You can see how sometimes there might be more or fewer holes, or bigger or smaller holes, at some times compared to others, depending on the behaviour of the mask-wearer. And if the holes in one layer line up with the holes in another layer – a virus can get through.
Prof Reason proposed this model as a way to layer multiple imperfect defences so as to maximise the chance of preventing catastrophic failure – the aviation industry has used it a lot. But catastrophe can also arise from acquiring SARS-CoV-2 infection. As we now know too well. Reason said while speaking about the model in an interview in Australia in 2005 …
…when you build defences against known hazards, you try to do the best you can but you build them in layers, but they’re never perfect. They’re like Emmenthal, they’re like Swiss cheese, they’ve got holes in. And so you can have several layers of cheese.
It’s like Sod or Murphy and the malign furies with the knitting needle trying to find the way through, and very rarely they do, but of course the holes opening and shutting and moving around, they’re not actually like Swiss cheese, and they’re being opened or shut by people at the sharp end who maybe making errors or by designers who fail to anticipate this particular trajectory of accident.
I also found it interesting that Prof Reason noted..
…he had in his head these two notions: the biological or medical metaphor of pathogens, and the central role played by defences, barriers, controls and safeguards (analogous to the body’s auto-immune system)
Leadership pressure to “get back to normal” or “live with the virus”
Something else noted by Reason, which I think we’ve seen a lot of, is the impact of leadership pressure (he highlights this in terms of management ) as a cause of big failures or aligned holes.
Pressures that include needing to meet deadlines and to cut costs. In the case of COVID-19, I’d argue a driving pressure was to get back to making profits and to restore individual freedoms at the expense of societal safety and protecting those who made our current lives possible; our parents.
Wherever restrictions were dropped while lots of cases still circulated (I don’t know where the percentage came from that suggested a level of safety at which opening was fine, but it was far too open to variability and interpretation), a surge was sure to follow. It usually only took a holiday or a seasonal change or life as usual to fan those embers into another bushfire (aka “Wave”).
One early COVID-19 failure was the inability to accept that it was the pandemic itself that could create economic hardship through a range of its impacts on our communities. When not reined in, it kept doing so.
This failure was fuelled by a narrative that the required (shorter period of) harsher restrictions to contain pandemic virus transmission was unachievable because it would cause too much harm. Instead, we have watched considerable longer-term harm befall many regions and regions that underwent the short-term pain, start bouncing back to a border restricted life-as-usual.
If 2020 taught us anything, it was that we were capable of a lot more than we knew. We could even eliminate transmission of an efficiently transmitting respiratory virus – something I didn’t think we could do up until this year.
My first view of Swiss cheese related to COVID-19
I first saw a version of this via @sketchplanator on Twitter sometime around October 4th (their image also links to a different version for the Cleveland Clinic, below).
I loved this version but it felt like it missed some specifics that would be useful to add for a broader understanding of what the public could do, and should expect to have done, to prevent the spread of SARS-CoV-2.
I tend to overload my own graphics and was conscious of that habit, but still thought more would be better. So I drew up my own version and tweeted it out.
My first version
I was sure I’d missed important bits out. And of course, the first version had a typo. Evolution ensued.
I next went to my wife and to specific people (thanks Jody!) and to Twitter as a whole, seeking thoughts and advice (which I sometimes did take, and sometimes didn’t). That can be a…hard process sometimes. But very worthwhile.
The red arrow was enhanced to highlight that when the holes (failings in any given layer/intervention) aligned, the virus could possibly get through multiple defences – so the more layers the merrier. And the tone of the text under the cheese was flipped from negative to positive.
Prof Reason further described the many holes…
though unlike in the cheese, these holes are continually opening, shutting, and shifting their location.
Version 2 added a layer
October 15th saw a vaccine slice and combined contact tracing with fast and sensitive testing as well as hand and surface cleaning.
One of the freed-up layers was used to introduce isolation and quarantine and a new government Comms and financial support slice. This layer should include education in its messaging, to make sure everyone is on the same page in understanding what’s happening and why.
Government support should include the measures necessary to support workers who have to be away from work to prevent spread and who will thus lose vital income. It also includes support for those paying and collecting (to prevent eviction or rises) rent and support for business.
The suggestion was raised that there be a break glass “lockdown” button. It wasn’t added then but see version 4 below).
October 24th saw the third version emerge. Its arguably greatest addition - "the misinformation mouse” – a symbol of the erosion caused by niggly sowers of lies or simply non-expert opinion they feel must be shared with or shouted at the world. The unfortunate consequence is that these “mice” while being completely wrong, provide often a sort of comforting wrongness that rings true for those who are simply overwhelmed by the scope and complexity of a pandemic’s many facets.
These poisonous pests can undermine actions needed to save lives and protect health and livelihoods. They are not your friends. In some instances, those views are held by national leaders and the consequences are many more preventable deaths than might otherwise have been attributed to the less influential but usually louder individuals or groups.
Version 3 also added two more slices including limiting time inside, the need for ventilation, preferring outdoors to indoors and the introduction of air filtration; all to combat an aerosol-borne virus. It also added the need to stay home if sick to the physical distancing slice.
And thanks to the University of Queensland crew, who had already adapted a version for internal communication with students, I added their rough groupings of personal and shared responsibilities.
Using Swiss cheese: a concept not dogma
As with all aspects of this infographic, the groupings are not a mandate (there is overlap between layers and groups) just as the layers are not in a specific order of importance nor are the number of holes representative of the degree of dodginess of any given layer.
The overall idea is to convey a concept. Which it seems to have succeeded in doing.
This is newly made for this blog – it includes a layer called border controls – because, without this, it’s almost impossible to imagine seriously getting a pandemic under meaningful and long-lasting control in a given region – if that’s your aim.
We’ve seen the world over that when borders are open there is a constant incoming source of new virus (including novel variants which create further headlines and fear) and it, along with the clusters they trigger, can easily overwhelm contact tracing and laboratory testing capacity. Once those have gone under, it’s the wild west of transmission.
What worked well for Australia and New Zealand?
If I was pushed to create a personal (this is my opinion), ranked, Top 10 of what was needed to prevent the spread of this or any future – perhaps more severe – respiratory virus pathogen, then my current list would include the need for:
- Early communication
- Regular communication (look to Victoria, Australia for its example of daily loooong briefings)
- Authentic communication (look to Victoria & Queensland)
- Anti-misinformation communication – flexible, nimble, animated and relatable
- External border controls
- Internal border controls
- Lockdowns (adequately financially supported and with good mental health support as well as constant reminders to see to normal health issues – cancers, heart disease, you name it)
- Testing capacity must meet turnaround times of <48 hours and have in place additional surge capacity
- Contact tracing
- Quarantine and isolation that is backed up by public health direction supported by law (protocols should be in place before they’re needed)
Making the cheese accessible to all
Apart from tweeting the image and it being picked up by media all over the world, I quickly placed the image (.png format) and an editable vector graphic (.svg format) version of it onto my Figshare page.
This page includes a CC BY 4.0 open-access license which lets you share and adapt the image and only asks for credit for the source and an indication of any changes you make.  Thanks to a number of people – but especially Dr Prital Patel – for creating over two dozen different language variants that you can grab from that site and share among your own communities.
Plus a graphical/icon version and a mug-sized version in case you want to make your own version to have in the office (see my how-to on Twitter, if the Redbubble mug doesn’t do it for you).
- The contribution of latent human failures to the breakdown of complex systems
- Absent-mindedness/Risk Management
- Human error: models and management
- The Swiss Cheese Respiratory Virus Defence
- Swiss Cheese Respiratory Virus Pandemic Defence by VirolDownUnder
This article was originally published on Virology Down Under, a blog run by renowned infectious diseases expert Associate Professor Ian Mackay. Business News Australia republished the piece with permission from Mackay.
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