A popular model in risk management across domains as diverse as aerospace, healthcare, mining, and manufacturing, the Swiss Cheese Model gained a new audience when it was repurposed to represent a multi-layered COVID response.  The original Swiss Cheese Model used the metaphor of cheese slices to represent lines of defences spanning personal and organisational factors. Losses and impacts can be prevented through this multilayered approach and will impact if/when a hazard travels through an alignment of holes.   TO ERR IS HUMAN. This model was developed in the 90s, at a time when safety tended to be viewed through the prism of personal errors. This led to a focus on shifting personal behaviours through methods such as training, confronting campaigns, and/or disciplinary action. In that context, James Reason and his Swiss Cheese Model was part of a movement that took a more holistic approach, incorporating a system view beyond the personal.  This systems approach was based on an understanding that “to err is human”, and that errors were inevitable. Reason identified unsafe acts as either intended (violations or mistakes) or unintended (mistakes again, with the addition of slips and lapses). Indeed, Reason began his exploration into this area when he distractedly put cat food into his teapot – see Origins below for more.  INEVITABLE HOLES.  Industries and workplaces have created their own versions of this model, typically labeling the layers, or cheese slices in the metaphor, to span personal and organisational factors. Reason accepted that each layer of defence will have unintended holes, where things will inevitably go wrong, and classified them as:  Active failures: these are unsafe acts committed by people typically at the ‘front end’ of the process. For example, at Chernobyl, the operators violated procedures by turning off safety systems. However, rather than being isolated personal factors they can generally be traced back to underlying systemic issues.  Latent conditions: these are the ‘resident pathogens’, as Reason described them, that are the system or back end challenges, including elements such as design decisions, faulty processes, or equipment issues.  In many cases, hazards will make it through one or even several layers, but will remain contained and have a minimal impact — though ideally these breaches will be uncovered, tracked, and serve as feedback for continual improvement. The viewable impact arises when the holes in all defence layers are aligned, allowing hazards to break through and impact people.  APPLIED TO COVID.  More recently, Australian Virologist Ian Mackay adapted the Swiss Cheese Model to represent a defensive response to Covid. It points to the fact that each intervention is imperfect, but together they can create a strong defence. After posting earlier versions on Twitter, Mackay added a ‘misinformation mouse’ which, when left unchecked, will eat away at the defences. See the In Practice tab for Mackay’s original diagram.  IN YOUR LATTICEWORK.  The Swiss Cheese Model can be used as part of a risk management strategy that can incorporate the Risk Matrix, and the idea of building multiple defences aligns with Margin of Safety. The acknowledgment of fallibility connects to the irrationality of humans explored through Fast and Slow Thinking.  Understanding unsafe acts will likely lead you to root cause tools such as the Fishbone Diagram and/or 5 Whys, and often personal safety issues can be related back to Psychological Safety. Finally, the resonance of this model connects with Aristotle’s Rhetoric and the power of metaphors.