The Swiss Cheese Layer of Defense

Carter Mecher MD -- Medical Advisor
August 13, 2021

Three words describe the 2007 Community Mitigation strategy developed by the United States for pandemic influenza—early, targeted and layered. Let me explain what is meant by layered. The individual measures such as isolation of the ill, quarantine of contacts, closure of schools, and social distancing in the community and in the workplace are ineffective if implemented alone. Each intervention is only partially effective. It is the layering of these interventions that is important in slowing disease transmission. The combination of the interventions is much greater than the simple sum of the individual interventions. The combined effect of these interventions on slowing disease transmission is nonlinear. The whole is much greater than the sum of the individual parts.

Back in 2006 I borrowed a concept developed by James Reason—a Swiss cheese model of accident causation. I was familiar with this model as it applied to patient safety in healthcare. The idea was a layered defense of partially effective strategies -- each layer had holes like a piece of Swiss cheese. Occasionally the holes would line up just right and you would have an accident or a serious patient mishap.  Patient safety focused on when the holes were aligned. It was noted that catastrophic accidents and deadly patient errors are relatively uncommon because of that layered defense -- those multiple layers of Swiss cheese. But even those relatively low rates are unacceptable in medicine. Rather than focusing on the occasional lining up of those holes of Swiss cheese, I chose to focus on how often those holes offset one another and prevent bad outcomes.  By simply applying that Swiss cheese model -- the layering of only partially effective interventions one atop another so that the holes in the Swiss cheese mostly cover one another -- you could effectively slow disease transmission. There wasn't a need to have perfect coverage like you would for preventing the rare serious accident or serious patient mishap. The objective was to slow disease transmission enough so that the reproductive number (R) would fall below 1 and the outbreak would extinguish itself. It was liberating -- well beyond pandemic preparedness and response -- to realize one could solve a vexing or intractable problem by layering imperfect strategies vs. wracking your brain to come up with a perfect or elegant solution.

One of the lessons of this delta wave, is the country's failure to mount a layered defense.  Our strategy pretty much depended upon a single piece of Swiss cheese -- vaccination. It just turned out that for delta, the holes in that single piece of cheese were much larger than anyone thought.  We did employ other measures (half-heartedly), including isolation of the ill, quarantine of contacts, and the use of masks.  However, those slices were much more hole than cheese. Our under-testing and low case ascertainment (fueled by large numbers of asymptomatic individuals represented by the large numbers of unvaccinated children and young adults with high rates of asymptomatic disease as well as the high rates of asymptomatic disease in vaccinated individuals) meant that the majority of those who were infected and infectious did not isolate, and the majority of contacts were not quarantined. We also had a stuttering response in terms of face mask use coming at the worst possible time -- in May, with the arrival of the Delta variant, vaccinated individuals were told they no longer needed to wear masks and non-vaccinated individuals interpreted that signal to abandon the use of masks as well.  A number of states even abandoned the use of masks in schools.   Given the ongoing battle within the United States regarding face mask use, this layer is much more hole than cheese. It is hardly even a slice of cheese during this wave. Really a slice of air.

The lessons we learned back in 2006 were that we needed an "early, layered, targeted" strategy.  We needed a defense in depth to defeat an exponential threat. These measures are agnostic with regard to severity --they are focused on transmission.  Vaccination was and is extremely important. Through vaccination we have prevented serious illness, hospitalizations and deaths during this wave. The United States just couldn't achieve a high enough level of vaccination to prevent the numbers of the seriously illness, hospitalizations, and deaths that we are now experiencing.  Our reliance primarily on a single layer of defense strategy to defend the United States against the Delta variant will prove to be our Maginot Line.

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