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.
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.
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, fiddling with, the mask
- Sharing masks
- Not using the right type of fabric or enough layers for your mask
- Not wearing the best type of mask – an N95 or better
- Not wearing the mask wherever there is a risk – which may be everywhere unless better air cleaning is introduced at shared/common indoor spaces like gyms, cafes, cinemas churches, and classrooms
- 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 only one of the layers! Every layer has imperfections and is subject to more because humans take shortcuts and are inherently lazy at times.
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 a 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.Absent-mindedness/Risk Management 
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)The contribution of latent human failures to the breakdown of complex systems 
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 include needing to meet deadlines and cutting costs. In the case of COVID-19, I’d argue a driving pressure was to get back to making profits and restoring 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.
Also note the heavy mention of the “layered” use of interventions to mitigate pandemics in the CDC’s 2007 document, Interim Pre-Pandemic Planning Guidance: Community Strategy for Pandemic Influenza Mitigation in the United States—Early, Targeted, Layered Use of Nonpharmaceutical Interventions.[6,7]
My first version
I was sure I’d missed important bits. 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.Human error: models and management
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 the 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 businesses.
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. It’s arguably the greatest addition – the misinformation mouse” – a symbol of the erosion in trust, action and progress caused by niggly sowers of lies or simply non-expert opinions 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.
This version was posted to Twitter in September 2021. It added a communication slice to emphasise how essential communication is to prevent outbreaks. Without all of your communities involved, energised and on board, there is little chance you can minimise risk and disease.
It also adds a mouse to the vaccination slice and combines mask touching with hand hygiene. It may not play much of a role for an airborne virus, but the precautionary principle suggests it’s better to have this in mind than to leave it out. Also, hand hygiene is generally good for preventing the transmission of pathogens. It also hurts nothing to keep this habit up.
“Lockdowns” are rebadged as stay-at-home orders, and there is an addition of noting CO2 levels alongside crowding-it doesn’t indicate the likelihood that virus is in the room, but high levels tell you that if a respiratory virus is in the air, it will be lingering for longer because air is not be exchanged with fresh outside air (it may be being filtered though).
This version was posted to Twitter in October 2022. I added more content – so it’s way too busy now – but it has also a different phase of the pandemic. The message doesn’t have to be ‘at a glance’. We can take some time to read and learn. I think this better captures the many things that can be done to add layers of defence against transmission of airborne respiratory virus like SARS-CoV-2, influenza, respiratory syncytial virus (RSV), human metapneumovirus (MPV), parainfluenza viruses (PIVs), enteroviruses (EVs), parechoviruses (PeVs) rhinoviruses and more.
One of the underlining text boxes has been changed from ‘Shared & Personal Responsibilities’ to ‘Government/Corporate Responsibilities’. This is to reflect that doing en masse things like purifying the air in restaurants, classrooms, gyms, cinemas, churches and bars keeps safe hundreds of people at once. This is much more effective and better ‘bang for buck’ than individuals wearing crappy masks, badly and at the wrong time.
‘Wrong time’? For example, I wear my mask around my staff and through the hospital all day, but then I come home and take it off only to be infected by my child who picked the virus up at school or my partner from their exposure at the gym. Or at a restaurant because try though I might, I still suck a steak through a piece of melt-blown fabric.
Sometimes we have to take our masks off while still living life but the minute we let our guard down is the minute we can be infected.
Better ‘cleaning’ of the air at mass gathering sites like those listed, will more effectively interrupt airborne virus transmission at scale, putting less pressure on masks to work perfectly all the time (they can’t) and therefore reduce many individual burdens. It’s actually pretty simple if you think about it. Demand that sites where people gather indoors purify their air because then it makes your life simpler and less prone to illness. Even if you don’t care about the deaths that surround you today – you will as you age because it will be you one day, that no-one seems to care about.
This version also:
- Elaborates on the type of mask – fitted and N95-or-better
- Notes the need for science-informed leadership
- Asks for reporting on virus data (not just SARS-CoV-2; stop hiding public data!) and CO2 levels
- Reminds us that we are responsible for getting vaccinated and boosted
- Reminds us to limit risky indoor time if the site is crowded
- Pleas for paid sick leave
- Notes the absolute need to keep funding improved vaccines (we need mucosal immunity).
- Removes the mask from the person as masks are among the layers.
- Added more misinfo mice because, the world
- Recognises that virus can get through most layers – but the more layers you have, the lower the viral load you are likely to be exposed to, and a lower infectious dose can mean milder illness which might mean less long-term health damage.
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 – https://figshare.com/articles/figure/The_Swiss_Cheese_Respiratory_Virus_Defence/13082618. This page includes a CC BY 4.0 open-access license which lets you share and adapt the image and only asks you to credit 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.
Coffee, tea and cheese
You can also add the graphic to a mug of your own choosing, or you can buy one ready-made (see below). To add to a coloured mug offered by some companies, try following this process:
- Download the .png (also from above👆) or the .svg file from my Figshare Swiss Cheese page (will take a little searching, try ‘swiss 5.4’) as there are all versions and different language versions on there.
- If you just want to slap on the image as is, use the .png and work with your mug supplier. I got mine from Vistaprint simply because they had yellow-coloured mugs available when I was looking.
- If you want to edit the image – go ahead. I installed a font called Reprise Script first and downloaded Inkscape to work with it.
Merch? Ian! You cheese shill!
The image is also available on my newly created Redbubble store  placed onto a bunch of different items you can buy if you are so inclined.
Any after-tax profits (the price of each item includes a 10% profit) will be donated to a charity to be decided by a Twitter poll in 2021. I’m not in this to make any personal profit. You can see the progress on these donations if you visit this thread which I will keep updating.
So that’s it – probably what I’ll end up being best known for in 2020 – a stack of cheese!
It’s been amazing watching this one image take off and help people understand the complexities of preventing respiratory virus transmission and disease.
- 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
- A ‘Swiss cheese’ approach can give us a second chance to contain Covid-19
- Added version 5.4 and explanations.
- Added a ‘How To’ get the graphic onto a coloured mug at Vistaprint
- Added link to show the Redbubble shop profits being sent to MSF Australia.