The Swiss cheese metaphor for covid-19 precautions

Now that Trump is a lame duck, the. Centers for Disease Control seems to have become more activist and not looking over its shoulder to see if he approves of their recommendations. In the light of record hospitalizations, they have issued the four basic preventative measures that all of us can take, including for the first time asking people to wear masks at all times whenever they leave their homes, moving away from Trump’s ambivalent attitudes to mask-wearing and moving closer to Joe Biden’s stance on it.

* Wear a mask.
* Wash your hands.
* Avoid crowds.
* Stay 6 feet from people who don’t live with you.

The CDC on Friday for the first time recommended that people wear masks at all times when they’re not home, saying the coronavirus pandemic in the U.S. has entered a phase of “high level transmission.”

The guidance, included in a new report, advised state and local officials to impose mask mandates for indoor settings as part of broader mitigation efforts to control the spread of the virus. It echoes President-elect Joe Biden’s call for mask orders that a number of red state governors have rejected.

The agency’s new report includes a number of other public health measures local and state governments can take to help get the pandemic under control, including issuing policies restricting nonessential indoor gatherings, like indoor dining, and beefing up contact tracing efforts.

There is no guarantee, of course, that doing any one of those things by themselves will protect you. You should do as many as you can. By doing all of them, you can make the probability of getting infected quite small. Since many people do not have an intuitive grasp of probability, I am passing on a metaphor that I heard an epidemiologist give that captures the essence of this idea. He says that each preventative measure is like a slice of Swiss cheese with randomly distributed holes that can let the virus pass through. But the more layers of cheese one has, the more likely it is that the holes in one slice are blocked by cheese in the others, thus greatly reducing the chance of the virus getting through.


  1. says

    The problem is, people are not generally used to think in terms of probabilities. In my experience, most people think that if something does not work 100%, then it is not worth it. Even people with university education.

  2. Who Cares says

    It is not just not used to think in probabilities. We can’t handle large numbers, exponential growth, new data changing recommendations, non absolutes or that some measures are less to protect you then they are to protect others from you.

    Which is why the damage that Trump did to the CDC reputation and messaging is so damning.

  3. says

    The problem is, people are not generally used to think in terms of probabilities

    That’s part of the story of the life of a computer security consultant.
    Me: “It’s a bad idea to X!”
    Client: “Why? It hasn’t happened yet!”

    I used to try to come up with comparables, i.e.: “if this thing fails your likelihood of being compromised is about the same as your likelihood of dying if your car hits a semi-trailer at 80mph.”

    Client: “Why? It hasn’t happened yet!”
    Me: /facepalm

    All those municipalities and hospitals in the US that are getting cryptolocker? Me and my friends warned them and told them how not to get it. But. they hadn’t gotten it yet.

  4. machintelligence says

    Some folks won’t believe something until it affects someone they know.
    Some folks won’t believe something until it affects them,
    And then there are the holdouts who won’t accept something even if it happens to them.

  5. Matt G says

    The exponential growth thing is a biggie. Think about the grains of rice on a chessboard story. Really, anything that isn’t easy, intuitive and consistent with orthodox religious belief.

  6. Jean says

    People won’t wear a mask but they will buy lottery tickets. Talk about not understanding probabilities…

  7. Dunc says

    What do you think the overlap is between people who think masks are pointless because they’re not 100% effective, people who don’t think covid is a big deal anyway because the chances of a young, healthy person dying from it are pretty small, and people who are very worried that the vaccines aren’t safe because they’ve been pushed though testing faster than usual?

  8. says

    @Cryp Dyke, I had actually better success explaining this to my sister in law than to one of my friends from university. My sister in law has only vocational education and she grasped both exponential growth and Occam’s razor with regard to the origins of the virus really well, whereas my friend with university education fell for the most ridiculous conspiracy crap there is.

    Unfortunately, there really are people to whom explaining things they already have formed opinions about is of no use. And intelligent people are sometimes better at rationalizing their preconceived notions than at understanding reality.

    That does not however mean that explaining something to people is meaningless and I was not saying or even implying that. It only means that no matter how good the explanation, it won’t be 100% successful and sometimes not even mostly succesful.

  9. prl says

    In the Australian state of Victoria, (population ~6.7 million), a second wave of Covid-19 started in mid-June (there were no cases on 6 June). By 8 August, the 14-day average number of cases was 450/day. Since 10 November, there have been no new cases of Covid-19 in Victoria.
    On 2 August, Victoria went into stage 4 lockdown. The measures in Metropolitan Melbourne (the capital, where almost all of the cases were) were:
    * Stay at least 1.5 metres (just under 5 ft) away from others.
    * Face coverings mandatory.
    * Curfew 8pm-5am.
    * Outside the curfew, people only be allowed to leave the house for: purchase food and necessary supplies, exercise, give or receive care, or work.
    * Residents restricted to five kilometres from their home, except to give or receive care.
    * One person per household will be allowed to go shopping once per day.
    * Only essential retail outlets may remain open.
    * Outside exercise limited to once per day for up to one hour, in groups no bigger than two.
    * Recreational activities including tennis, golf and fishing not permitted.
    * Funerals remain limited to 10 mourners.
    * No weddings allowed.
    * Religious ceremonies and worship services limited to five people.
    * An AUD1,652 (~USD1200) fine for breaches of lockdown measures.

    The Victorian borders were also closed to all other states and to international travel.
    Those measures got the number of new cases down from its peak on 8 August to 0 cases from 10 November, about 13 1/2 weeks.

    IMO the measures that Mano has suggested are necessary, but probably not sufficient, to really bring Covid-19 under control.

    Other parts of Australia had restrictive measures in place during the first wave, and there were various other state border closures, but nowhere else had lockdowns as strict as the Aug/Nov lockdown in Victoria.

    Australia is open to limited numbers of overseas travellers (Victoria re-opened to overseas travellers just a few days ago).

    There are currently 44 active cases of Covid-19 in Australia (population ~25.7 million). There has been 1 locally acquired new Covid-19 case in the past 14 days (a quarantine hotel cleaner appears to have caught Covid from overseas flight crew in quarantine), and a total of 9 new cases in the same period (the rest almost all in quarantine).

    The link to the “full list of restrictions” in the linked article points to current restrictions (so-called “Covid normal”) in Victoria, not to the stage 4 restrictions.

  10. prl says

    There has been 1 locally acquired new Covid-19 case in the past 14 days (a quarantine hotel cleaner appears to have caught Covid from overseas flight crew in quarantine), and a total of 9 new cases in the same period (the rest almost all in quarantine).

    Those numbers are wrong. It’s 9 locally acquired cases nationally in the last 14 days, and 205 cases from overseas (14 day average of about 15 cases/day).

  11. Holms says

    #1 Charly
    This is a very frequent response whenever conversations about gun control flare up. “Firearm ID checks / purchase waiting periods / etc. won’t stop 100% of firearm misuse, so we should just not bother!” And then when you explain to them that this is applicable to every area of law, they usually move on to the next silly argument without acknowledging the point.

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