Layered mitigation
Swiss Cheese Defense Model
Each protective measure has “holes” (imperfections), but stacking multiple layers dramatically lowers risk. Select the layers in place and estimate compliance to see cumulative protection.
Estimate how consistently layers are applied (e.g., 80% = most people comply most of the time).
Selected layers
4
Compliance factor: 80%
Residual risk after layers
13.6%
Baseline risk × 0.14
Additional layers (e.g., testing or ventilation) could further mitigate risk in high-prevalence settings.
Residual risk multiplies baseline exposure risk (e.g., 10% of baseline).
How to Use This Calculator
Inventory existing protections
List the layers currently deployed in your setting (schools, offices, healthcare facilities).
Estimate compliance
Consider real-world adherence. Lower compliance increases residual risk even with multiple layers.
Add layers to plug holes
Target the largest “holes” by introducing ventilation, high-quality masks, or testing to reduce overall risk further.
Formula
Residual risk = Πlayers (1 − Layer effectiveness × Compliance factor)
Effectiveness values are illustrative (0–1). Compliance factor adjusts each layer for real-world use.
Overall protection = 1 − Residual risk.
Full Description
The “Swiss cheese” model, popularized by James Reason and adapted for COVID-19 by Dr. Ian Mackay, illustrates that no single intervention is perfect. Each layer (vaccines, masks, ventilation, testing, etc.) has gaps, but combined layers block most transmission pathways. This calculator helps visualize how layers interact multiplicatively to reduce risk.
Tailor the layers to your context, improve adherence through communication, and remain agile as guidance evolves. The model supports a culture of shared responsibility and redundancy.
Frequently Asked Questions
Do percentages here reflect real-world studies?
Values are approximate for educational use. Adjust effectiveness based on current literature and setting-specific data.
What if compliance differs by layer?
You can adapt the calculator to assign layer-specific compliance factors. Here we use a single average for simplicity.
Is ventilation as important as masking?
Ventilation reduces aerosol concentration. Combined with masks, it offers synergistic protection, especially indoors.
How do we account for variant changes?
Variants with higher transmissibility may require additional layers or higher compliance. Update assumptions as new evidence emerges.