A privacy respecting track and trace app to fight Corona is possible and effective

An app to track and trace infections seems to be a promising way out of the lockdowns. Tracking the contacts of infected people is a main strategy to fight this epidemic as long as we do not have a cure or vaccine. It is the main strategy used in South Korea and they are able to keep the number of new infections below 100 per day with it.

When the virus spreads more widely, like in many countries who did not take the virus serious enough soon enough, it becomes difficult for the health departments to track and trace so many people. In addition, a part of the contacts will not be known to the infected person and will thus not be tracked; for example, someone sitting next to you on public transport or in a restaurant.

How the app works

For the last case South Korea uses GPS information from mobile phones. I am not comfortable with the state having all that location data, but fortunately there is a better alternative. This is a great cartoon explaining how contact tracing can be done fully respecting privacy. The short version of this is below.



The Chaos Computer Club (CCC), German's most reliable technology activists, explain the conditions to make this work and have promised to warn about bad apps. I am happy use such an app and will listen to the CCC for advice. The CCC is comparable to America's [[Electronic Frontier Foundation]].

Let's hope data brokers Google and Apple getting involved does not mess this up. At least one group of scientists who started this approach are hopeful Google and Apple will help. We need many people participating, so we need something everyone can embrace.

How effective it is

A recently published study published in Science claims that such fast contact tracing could be as effective as a lockdown if 60% participate & 60% heeds its warnings.

To compute this they first estimate how the virus spreads; this paragraph can be skipped if you are not interested in the scientific basis. They estimated how long the incubation time is (5.5 days). On average it takes 5.0 days between one infected person and the next to show symptoms. So the moment someone gets ill, the people they have infected have started infecting other people. They estimate that on average 1 person infects 2 others. (This is a low value, other studies tend to find between 2 and 3.) The direct transmission from a symptomatic individual to someone else ("symptomatic transmission") explains 0.8 infections of those 2 infections. So even if if we theoretically would remove this fully the number of infections would still grow exponentially. Infected people infect 0.9 people before they show symptoms ("Pre-symptomatic transmission"). People without symptoms infect 0.1 people ("asymptomatic transmission"), while "environmental transmission", infections where people did not meet, account for 0.2 infections.

So it is important to be fast. This is the advantage of the app over a health departments trying to reach people by phone and email. Still it is worthwhile to both do manual and app tracing. A person from the health department calling you telling you your friend or colleague is ill and explaining how quarantine works is likely more effective than a notification by your phone. For this manual work to be effective we need to get the number of new infections down.

The speed of the testing is an important part of this strategy. It will thus work better in countries like Germany with a strong testing program than in America where much less testing is done, which in the short term makes the numbers look better, but does not make the situation better. The paper also studies how effective it would be if people with symptoms can warn people before being tested. This is naturally faster, but false warnings triggered by hostile actors can abuse the system. To avoid this one can tie the app to test results, where the health care providers can give the app user a code in case of a positive test.

When the app warns someone that they have been in contact with an infected person, this person will have to go into quarantine. This will work better in a country with paid sick leave and when the government gives the warning of the app the same status as "sick certificates" from the doctor.

The proximity detection by Bluetooth is far from perfect, so there will be false positives, but I would argue that that is still better assuming everyone had contact with an infected person and putting all of society on lockdown.

Enough people will have to participate. Fortunately it does not have to be all, apparently 60% is already enough. The privacy invading app of Singapore only has a take up of 10 to 15%. I would personally not use such an app, I'd rather take a small risk dying than giving a government really dangerous powers, while I would be happy to use the above described one. So I would expect the adoption of a decent app to be higher.

One would install the app to help others, so this may work less well on countries where the ruling class has pitted groups against each other to solidify their power. Ross Anderson from the UK is pessimistic about the adoption of such an app. I am quite optimistic. But we will have to do the experiment. Do note that when reading the second opinion of Anderson that I feel he does not accurately describe how the app would work; part of my text above is based on such misunderstandings others may have.

Prof. Dr. Christian Drosten, one of the main virologists in Germany who specializes in emerging viruses, thinks the app could work to reduce infections. In a recent podcast of the public radio channel NDR Info he talked about the app:

This is a study from the group of Christophe Fraser, certainly one of the best epidemiological modelers. It's a very interesting study, I think. It's published in Science. ...

The main outcome of the study is that you are too late with a simple [manual] identification of cases and contact tracing, because the whole thing depends on identifying symptomatic patients. So it really comes down to the last day. ...

And you can say in a nutshell, if the epidemics ran at the same speed as in the beginning in Wuhan ... then you could already lower R0 below one. This is amazing.

There are a few caveats on that. It is then said that in reality the speed of propagation in Europe is already faster than it was at the beginning in Wuhan. There are certainly several reasons for this. Population density, behaviour of the populations, but also how far the infection has already progressed. This of course makes it even more difficult again, so that a higher degree of cooperation among the population is actually needed. ...

You could combine such an App, for example by other general factors that reduce the transmission of the infection, such as wearing masks. ...

[The study models a situation where] there is no general lockdown. Companies can work, schools can teach, everything can work, but not for everyone at all times. There will come a time when you have this message on your mobile phone: "Please go into home quarantine." If you could then show this and your employer would say: Well, that's how it is, home quarantine this week. Then I find, that is at least a very interesting model one should not refuse thinking about.

Drosten can naturally only judge the effectiveness. The Chaos Computer Club (CCC), German's most reliable technology activists support the technical concept. It naturally depends on implementation details and while they will not recommend an app, they have promised to warn about bad apps. I will listen to the advice of the CCC.

Electronic Frontier Foundation makes clear that a trace and track app can only be part of a package of measures and rightly emphasise the importance of consent.

Informed, voluntary, and opt-in consent is the fundamental requirement for any application that tracks a user’s interactions with others in the physical world. Moreover, people who choose to use the app and then learn they are ill must also have the choice of whether to share a log of their contacts. Governments must not require the use of any proximity application. Nor should there be informal pressure to use the app in exchange for access to government services. Similarly, private parties must not require the app’s use in order to access physical spaces or obtain other benefits.

Individuals should also have the opportunity to turn off the proximity tracing app. Users who consent to some proximity tracking might not consent to other proximity tracking, for example, when they engage in particularly sensitive activities like visiting a medical provider, or engaging in political organizing.

A German conservative politician wanted to force people to use the app. He did not have a good day on social media. Well deserved. That is the most effective way to destroy trust and in times of Corona we need high compliance and thus solutions that have broad support.

The German National Academy of Sciences, Leopoldina, recommends three measures to replace the lockdowns. 1) Such an app. 2) Massive testing. 3) Everyone wearing simple masks in public. (In German.)

In the Netherlands, Arjen Lubach asks many questions on how such an app would be used. (video in Dutch.) Would your boss be allowed to force you to use such an app? Would this be a condition to use public transport? Would a restaurant be allowed to require customers to use an app? Would you be forced to share your random numbers when you find out that you are infected? Could you turn off the app? Could you ignore the warning of the app?

I had not considered many of these questions because I considered it natural to each time opt for the most free option and expect that that leads to much more people participating and thus to the largest effect. Any force to use the app would only make sense on a societal level. A boss or a restaurant has no advantages from such a measure, just like the users themselves only help society, not themselves.

My impression is that a main reason Germany got through this pandemic with only a blue eye is that the population was well informed, understood the danger, knew what to do and was very cooperative. It is relatively easy in science, but I have seen a huge part of people working from home well before there were any rules to do so. Meetings were cancelled well before the limits for the maximum number of participants went down to that level.

The alternative to so much compliance would be quite draconian rules and a lot surveillance and enforcement, leading to much more violations of freedom and economic damage. Thus I would expect that the best way to make the app a success is to respect the privacy of the citizens and respect their autonomy to make the right decisions. In countries were this is not possible, I am sceptical of the app helping much, except if they go full China and most countries do not have the enormous repressive system that would necessitate.

Where I do agree with Arjen Lubach is that we have to have this discussion now. It is not a matter of using the app or not, but how do we want to use it. We should have that discussion before we introduce it. Just like we should discuss all other measures and whether and when they can be relaxed. Even if we do not know exactly when yet, we can already discuss what has priority, opening school, shops, restaurants or car factories?


Disclaimer. In am just a simple climate scientist, not a virologist, nor an epidemiologist or encryption specialist. I had wanted to stay out of this topic and not pretend to be an instant Corona specialist, but the dumb people do not show such restraint and only few actual experts speak up. Those that do, report that they find it unpleasant. As a climate scientist I am unfortunately used to the well-funded hate mobs trying to bully others into silence and will not let myself be intimidated. Plus a large part of this post is about societal issues, where everyone should participate, not just experts.

Related reading

The position paper of the EFF is long, but worthwhile: The Challenge of Proximity Apps For COVID-19 Contact Tracing.

If anyone would like to get involved, there is a list with COVID-19 contact tracing projects around the world.

German public radio channel NDR Info makes a daily podcast with virologist Christian Drosten, on my blog you can find translations of parts of these interviews.

The German CDC, the RKI makes wonderful informative daily situation reports, in German and English.