Interventions That Work
Learning Objectives
Students will be able to:
- Identify different categories of infection control interventions
- Explain the "Swiss cheese model" of layered protection
- Calculate combined effectiveness of multiple interventions
- Design a layered protection strategy for a given scenario
The Swiss Cheese Model
No single intervention is perfect—each has "holes" (failures). But when you stack multiple layers, the holes don't line up!
Virus trying to get through
↓
[Ventilation] ——o——o——— (some holes)
[Filtration] —o————o— (different holes)
[Masks] ————o——o (different holes)
[Vaccination] —o——o——— (different holes)
↓
Very little gets through!
Categories of Interventions
Source Control
Reduce what sick person releases
- Staying home when sick
- Masks on infected person
- Cough etiquette
Environmental Controls
Clean the air for everyone
- Ventilation (fresh air)
- Filtration (HEPA, CR boxes)
- UV air cleaners
Personal Protection
Protect the individual
- Masks (N95/KN95)
- Vaccination
- Distancing, time limits
Administrative Controls
Policies and procedures
- Testing and isolation
- Capacity limits
- Remote options
Estimated Effectiveness
| Intervention | Risk Reduction | Notes |
|---|---|---|
| N95 respirator | 90-95% | When properly fitted |
| Surgical mask | 50-70% | Depends on fit |
| Source masking | 70-90% | Sick person wearing mask |
| Doubling ventilation | ~50% | Cuts concentration in half |
| HEPA air cleaner | 30-70% | Depends on size and room |
| CR box | 30-60% | DIY option! |
| Vaccination | 50-95% | Varies by disease/vaccine |
| Staying home when sick | ~100% | Eliminates source entirely |
The Power of Layering
Interventions MULTIPLY, they don't just add:
Starting risk: 10% chance of infection
| Add Intervention | Risk Remaining | Calculation |
|---|---|---|
| Good ventilation (halves risk) | 5% | 10% × 0.5 |
| Add filtration (reduces 40%) | 3% | 5% × 0.6 |
| Add masks (reduces 60%) | 1.2% | 3% × 0.4 |
| Add vaccination (reduces 80%) | 0.24% | 1.2% × 0.2 |
Result: 4 imperfect interventions reduced risk by over 97%!
Activity: Design a Protection Plan
Choose a scenario and design a layered protection strategy:
Scenario A: School Cafeteria
- 200 students daily
- 30-minute lunch periods
- Can't mask while eating
- Limited ventilation
Scenario B: Indoor Concert
- 500 people, 2 hours
- Singing/cheering
- Winter (windows closed)
- Moderate budget
Your Task:
- List 3-4 interventions for your scenario
- Explain why each helps in this specific setting
- Estimate combined effectiveness
- Identify limitations (what can't you control?)
Quick Check
Question: A school has masks (50% effective) and good ventilation (50% effective). What is the combined risk reduction?
Show answer
b) 75%
Masks allow 50% through (0.5), ventilation allows 50% through (0.5).
Combined: 0.5 × 0.5 = 0.25 = 25% risk remains.
Risk reduction: 100% - 25% = 75%
Key Takeaway
No single intervention is perfect, but layered protection works. The "Swiss cheese model" shows how multiple imperfect interventions combine to provide strong protection. Environmental controls (ventilation, filtration) are especially valuable because they protect everyone automatically—no individual action required.