Imagine multiple Swiss cheese slices with their holes and all, each lined up one after the other. When the cheese is stacked, it is hard to see what’s on the other side. However, when you take some slices out of the stack, it becomes more visible. When applying this analogy to the COVID-19 pandemic, the layers can signify social distancing, masks, hand-washing, refraining from touching your face, avoiding crowded areas, testing and tracing, ventilation, government messaging, quarantines, and vaccines.
The idea came about when Ian M. Mackay, a virologist at the University of Queensland, created an infographic of the Swiss Cheese Respiratory Pandemic Defense, noting our collective responsibility during the pandemic.
A new version with colour & division inspiration from @uq_news and strict mouse design oversight by @kat_arden (ver3.0).
— ɪᴀɴ ᴍ. ᴍᴀᴄᴋᴀʏ, ᴘʜᴅ (@MackayIM) October 24, 2020
It reorganises slices into personal & shared responsibilities (think of this in terms of all the slices rather than any single layer being most important) pic.twitter.com/nNwLWZTWOL
Originating from psychologist James T. Reason’s book Human Error, the Swiss Cheese Model of Accident Causation has been used to explain many disasters from the space shuttle Challenger Shuttle explosion to the Bhopal disaster, and the Chernobyl nuclear accident. There are many engineered or societal defensive layers to protect individuals from hazards. The model aims to explain that there is no single intervention that can be the answer to solve the virus, as each layer has its holes or faults. Instead, society should adopt multiple methods to stop transmission of the virus.
The layers are not in any specific order nor do the holes represent any real degree of fault. As mentioned before, the layers stand for each intervention, noting the ones that are a personal responsibility and those that are a shared one. The top factors to consider when mitigating the virus are early and regular communication, anti-misinformation communication, external and internal border controls, lockdowns, testing, contact tracing, and isolation.
The holes appear due to active failures, latent conditions, or a mixture of the two. Active failures are slips, lapses or mistakes by individuals in society. In contrast, latent conditions are the inevitable pathogens within politics or other systems.
In Reason’s analogy, he states that “active failures are like mosquitoes. They can be swatted one by one, but they still keep coming. The best remedies are to create more effective defenses and to drain the swamps in which they breed. The swamps, in this case, are the ever-present latent conditions.”
Mackay explains how little slips of action like that can create big disasters.
"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 behavior of the mask-wearer. And if the holes in one layer line up with the holes in another layer—a virus can get through,” shared the psychologist.
Aerosols can escape from an infected individual if they do not wear a mask properly, dispose of it incorrectly, fiddle with it, share masks, or use unsanitary ones. According to Dr. Trish Perl, the chief of the Division of Infectious Diseases at the University of Texas Southwestern Medical Center, masks reduce the risk of catching the virus by up to 70 percent depending on the type that is worn. Fabric masks may be able to block high viral droplets over 1 µm, but they can still leak out small droplets, infecting others around the mask-wearer. In contrast, an N95 mask, which is harder to find as the pandemic enters its 10th month, is highly effective. Since they use a melt-blown polypropylene fiber fabric, it equals 12 to 16 layers of cloth.
Countries worldwide are deploying contact tracing apps to keep an eye on the virus and infected individuals. Though the apps have made long strides in tracking the virus, there are still gaps to be filled. At least 60 percent of a population must download an app for it to effectively limit transmission. As a result, countries like Italy have seen the limitations of these apps.
“Even when we see cases in which apps have been downloaded a lot of times, and alerted a relatively high number of people, in terms of what that does for overall numbers and the resurgence of the virus, I think it’s important that we understand the limitations of these apps,” said Samuel Woodhams, a digital rights analyst. “Contact tracing apps are not going to be a silver bullet.”
Even when considering vaccines, such as Pfizer and BioNTech’s BNT162b and Moderna’s mRNA-127, there are some holes. Both vaccines are only 95 percent effective based on clinical trials. And, questions remain. Researchers are still looking into whether vaccinated individuals can transmit the virus if the body does not produce much of an antibody quickly. With much of the new data, researchers do not have a full grasp of the long-term effects.
There are two differences between Mackay’s model and that of Reason’s. The red arrow shows that when the holes are aligned, the virus could get through multiple layers. Since the holes are continually opening, shutting and shifting locations, it is more efficient to use numerous intervention methods. The misinformation mouse serves as a metaphor for the misinformation that can erode any of those layers and risk impacting our lives.
This model is not widely accepted without its share of criticism due to its limitations in understanding the full scope of the pandemic; however, it can guide society toward a better understanding of the risks and tools to combat the virus in these unprecedented times.