Coronavirus: The Swiss Cheese Strategy

How Any Country Can Learn to Dance and Stop the Coronavirus

  • How the US and the EU failed to control the virus, and how comparable countries succeeded.
  • How you can make sense of all the necessary measures with one simple idea.
  • Why the West’s testing and contact tracing is largely useless — and what they can do about it.
  • The questions that journalists and the People must ask politicians to keep them accountable.
  • How you can stop the virus in your own community, without the need of your government.
This is what has happened in Europe over the last nine months. Source: Tomas Pueyo commentary and annotations on map from innouveau

How to Dance

To keep the coronavirus at bay in your community, you have four layers of defence:

  1. Stop as many infections from coming in as possible
  2. For those that make it in, minimize the people they meet
  3. When they do meet people, minimize the likelihood that they will infect somebody else
  4. When they do infect somebody else, identify that infection quickly and neutralize it

1. The Fence: Keep Infections out

It’s common sense that if at the border you don’t stop people coming from infested areas, some will bring the virus in and cause outbreaks.

  • Walls: Ban travelers from infected areas.
  • Quarantines: Allow people to come in but isolate them for 4 to 14 days.
  • Checkpoints: Test people at the border.

No Fence, no defence.

What these people miss is that they might not be sufficient, but they’re necessary: They don’t work standalone, but without using them, it’s impossible to stop the virus. Tell me a single country that has been able to keep the number of cases low without a strong Fence. They don’t exist. Eventually, they get overrun. That’s why Japan, Taiwan, China, Hong Kong, South Korea, New Zealand and Australia all have strong Fences.

2. Social Distancing: Abstinence from Meeting Others

Some infected people will make it into your community. You can’t avoid it.

This article from Science Magazine is great at visualizing how the virus spreads in clusters and how backward tracing works to bust clusters
  • Ban gatherings above a certain number. The fewer the number of active coronavirus cases, the bigger the crowds can be allowed.
  • Target places that are likely to cause super-spreader outbreaks, such as prisons, elderly care homes, rehab centers, universities, or food packing plants. Help them prevent outbreaks with much tighter measures.
  • And when there’s a lot of community spread, close bars, clubs and restaurants, and even private parties.
  • Avoid lockdowns. And if you have to close establishments, schools should be the last ones to go, especially childcare.

3. “Contrafection”: Reduce Contagiousness When People Meet

The next layer of defence is to reduce contagiousness when people do meet.

Avoid crowded, confined, close, clamorous communication.

To methodically reduce contagiousness, we need to understand how the virus behaves and break down the steps that the virus follows to spread.

Source: El Pais — A room, a bar and a classroom: how the coronavirus is spread through the air. This article has great visualizations. Go read it.
  1. Avoid talking when in the presence of people from another social bubble. Definitely avoid singing or shouting.
  2. Spend as little time as possible with people from another social bubble.
  3. Prevent the virus from leaving the mouth and nose.
  4. If it does, circulate the air to prevent the particles from reaching other people.
  5. Create a hostile environment for the virus, so it dies even if it hangs in the air.
  6. Make sure it doesn’t reach somebody else’s mouths, noses and eyes.
The devil is in the detail, as usual. N95 masks are ideal. Surgical masks are great. Hand-made masks with a couple of layers of different materials also work. But single-material ones, like a bandana, don’t. It’s not enough to wear masks. Wear the right ones. This comes from the New York Times article: Masks Work. Really. We’ll Show You How

The coronavirus barely spreads outdoors.

That means nearly all activity outdoors should be allowed — except maybe things like concerts, sports audiences, or other big crowds. It also means that, if any business is able to move its activity outdoors, it should be able to open. Similarly, schools should strive to take classes outdoors.


The fifth point is something most people never really discussed. The virus has a hard time spreading with the right temperature and humidity.

Thanks to Autarkh and @LuisMateusRocha for the idea about the cave. Gif: Batman Begins, Warner Brothers, via @catchingcalcifer

4. Test — Trace — Isolate: Catch and Neutralize Infections

We’ve talked about preventing infections from coming into a community, avoiding meetings when they do, and avoiding contagions when there’s meetings. The last layer of defence is to identify infections when they do happen and neutralize them.

Infected people have an estimated number of contacts. What share of those is provably quarantined?

Contact tracers talk about rules like 90–90–90–90: reach 90% of infected people, gather 90% of their contacts, reach 90% of those contacts, and quarantine 90% of them. All of that, obviously within two days or less, otherwise the virus will have too much time to spread.

Photo: Joe Allison, via Anchorage Daily News

It’s too hard to isolate just a few people, so we’ll isolate them all.”

Governments such as those of Spain and France had no problem enforcing lockdowns with millions of fines. How come we never hear about fines for isolations and quarantines? It’s because they don’t exist.

The Swiss Cheese Strategy

These are the four layers to stop the spread of the virus: Fences, Social Distancing, Contrafection, and Test-Trace-Isolate.

Source: Tomas Pueyo, based on the Swiss Cheese model for safety incidents by James T. Reason. The concept has been circulating over the internet, with people like Ian Mackay tweeting about it. Others include this one from Jennifer Kwan, this one from Siouxsie Wiles and Toby Morris, the State of Guernsey, and many more.

Cheeses of the World

Every country’s strategy can be summarized in a combination of these defence layers.

What Should You and Your Country Do?

Fractals: Your Community Can Do It

So far, I’ve talked about countries because they tend to have the authority to apply all four layers. But they’re not the only ones. You can do it too.

Dancing Countries

If they do, the path forward is very clear: If they have an uncontrolled outbreak, apply a lockdown (layer 2) while they quickly ramp up their other layers:

  • They should announce a Fence that includes testing on arrival and soon after, with a quarantine in between.
  • Mandate a maximum size of people gatherings (e.g., 30 like in Sweden). Adjust it depending on the local prevalence.
  • Try to avoid other types of social distancing.
  • Mandatory masks when close to other people or in the same room with others. Masks are not necessary in the outdoors if you only have your family close by.
  • Goggles highly recommended indoors.
  • Allow most gatherings below a certain threshold of people — if they’re outdoors and with a minimum distance between people.
  • Mandate that indoors gatherings must have enhanced ventilation, with HEPA filters, higher temperature, and the right humidity.
  • Get enough testing for a positivity below 3–5% for PCR.
  • Announce the strategy for early detection.
  • Get around 2 contract tracers per daily case.
  • Require the publication of contact tracing efficiency: What share of infected people’s contacts are quarantined within two days?
  • Give resources to support people during isolations and quarantines with money, food, water, medicine and entertainment.
  • Make isolations and quarantines mandatory, tracked, and enforced.
  • Fine heavily those who break isolations and quarantines. Communicate these fines



2 MSc in Engineering. Stanford MBA. Ex-Consultant. Creator of applications with >20M users. Currently leading a billion-dollar business @ Course Hero

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Tomas Pueyo

2 MSc in Engineering. Stanford MBA. Ex-Consultant. Creator of applications with >20M users. Currently leading a billion-dollar business @ Course Hero