Germany's Covid contact tracing app is Open Source

Of course it's important and of the course the lower the false positive rate the better. But the argument remains: even with a high false positive rate, what's the net result? Maybe overall it's worth putting thousands of false positive in quarantine now than a whole city of millions two months later.

There are so many variables and known unknowns in this approach. I argue that even with all the well reasoned arguments against, at this scale and with this stakes they stay theoretical until we actually do it. Let's try at least once, and if it fails it's still knowledge gained.

If everyone of those has the app installed and has, on average, 100 registered contacts (so that’s about 98 false positives, if we assume that on average two of those lead to infections) that’s about 30,000 people being told to isolate every day.

If they are told to isolate for one week (should be sufficient, on balance) that’s around 200,000 people isolated at any one point in time.

If only 50% have the app installed that’s down to 100,000 people.

Sounds high but ok to me. Given how active the infection currently is in Germany I wouldn’t want false positives be higher than that – but somewhere in that high ballpark sounds fine to me.

Maybe at most a couple 10,000 sent into home isolation (a week should be plenty here, no need for 14 days – we aren’t shooting for perfect anyways) versus 81 million with strong measures.

Obviously the app isn’t the perfect solution here. Much more important is thorough and swift real contact tracing (followed by quarantine irrespective of diagnostics, so quite similar to how the app would work with many false positives). But I wouldn’t reject the app out of hand.

Say you need to inhale 1000 of a particular virus to get its disease, and talking normally with an infected person gives you 200/minute, so it takes you 5 minutes to get the disease.

You can get that 1000 by talking 5 minutes with one infected person, or you can get it by talking 3 minutes with one infected person and then shortly afterwards 2 minutes with another.

But what if the gap between those two is longer?

I'd guess that the first 600 you got get into cells quickly and start reproducing, but there are few enough that your immune system is able to handle them. The reason you need 1000 is that is the tipping point where the viruses can reproduce faster than your immune system can handle.

If that's the case, then after you get that first 600, how many you need to get on the next exposure would be 400 shortly after the first, but as the time between exposures increases that would increase.

The model here would be that you have a virus accumulator (you), being filled by exposure events and drained by your immune system, and you get sick if the accumulator reaches some threshold.

This makes me wonder if my approach to shopping during this pandemic is wrong. I've been doing a big shopping trip every 3 or 4 weeks, as opposed to before the pandemic when I'd pop in to the store 3 or 4 times a week and only buy a few things.

Those big trips involve being in the store for an hour or more (and it's not linear--the time to fill my cart is linear in the number of things I'm buying, but checkout time goes up faster because organizing my stuff at the self-checkout becomes harder). If someone else in the store is infected, I could potentially be around them enough to also get infected.

The old small trips only involve a few minutes in the store. If someone else is infected I'm not going to be in there long enough to get an infectious dose from them unless they do something like cough near me. With my mask, and care to avoid other people who are not wearing masks, my realistic risk from an infected person is less than an infectious dose.

If my immune system can clear that out in a few days, then I should be good to go for the next short trip. As long as the prevalence of infected people in my community is low enough that I'm not going to get an infectious dose in a single short trip, this should be a safe approach.

Another question: suppose I get that 600 dose, and no more, so my immune system handles it fine and I don't get sick. Am I spreading the virus during this time, or do I only start spreading after I get an infectious dose worth of accumulated exposure?

Edit: how does acquiring immunity fit in? If I get 60% of an infectious dose and then no further exposure until my body has dealt with that, do I get any immunity or does that only happen if I accumulate enough virus at one time to actually get sick?

By that model, if you wait long enough, the risk of getting infected by that one person you’re sitting next to approaches p, the fraction of people who are contagious (that person either is contagious, and you’ll get infected, or they aren’t, and you won’t ever get infected) and the risk of infection of serially being in the danger area of multiple persons (effectively taking independent draws of that 0.5% risk) approaches 1.0 (eventually, you’ll have encountered so many contagious persons that you’ll get infected, too)

Typically, you won’t meet enough people on the street to even get close to that 1.0, but it isn’t a given that that probability stays below p.

Things would be different if there were a minimal load (in a simple model, say, of V virus particles in a minute) needed to run the risk of infection. I can see this swing both ways, depending on various factors.

If you only shut down based on recently visited regions, you're likely to import cases anyways. With CoVID-19, it takes several weeks to identify an outbreak in a region (especially without extensive testing). By the time you shut down travel from a given region, you've probably already been importing cases for weeks.

The WHO has gotten a huge amount of flak for not calling for travel restrictions during this pandemic. But their recommendations were based on the ineffectiveness of travel restrictions,* according to previous research. They recommended screening passengers for symptoms, but also told countries to take measures to limit spread (like testing and contact tracing), on the assumption that countries would import cases.

* The WHO's recommendations were also based on the International Health Regulations, an international treaty that basically every country on Earth has joined. Most of the public is unaware of the IHR or what it says about travel restrictions, or of the history of the WHO (one of its founding goals was to stop countries from automatically shutting their borders whenever there was an outbreak, and instead to take rational, evidence-based measures).