4 ms·

From one of their strategy documents: "We believe that any likely decentralised model (because there are many) will have the following impacts in the UK: -Mov

From one of their strategy documents: "We believe that any likely decentralised model (because there are many) will have the following impacts in the UK: -Move the health response from ‘react to symptoms’ to ‘react to clinical test results’: We cannot currently find a way to manage malicious notifications, or possible amplification attacks, in a decentralised model without authentication. Consequently, notification must be uniquely tied to an authentic clinical test. This generates a dependency on the digital authentication of clinical testing … " I hadn't thought about that but that might be quite a big flaw in the Google+Apple approach. If central verification is required in that model, it introduces a delay step which might make it a lot less effective. SARS-CoV-2 seems to be transmissible very early relative to symptoms so this central notification step would need to incredibly quick (fast enough that I don't think you could wait for someone to be swabbed and PCR+ even with a very efficient testing service which the UK does not have).

g_p 6y ago
I posted this in reply to another comment, but yes this does appear to be the flaw of the Google/Apple approach. Assuming the incubation time is just over 5 days (seems a broad consensus here), and someone is at their most infectious in the day or 2 prior to this (not sure if this is yet broadly agreed, but some studies are suggesting this [1]) Imagine that person A is infected on day 1. On day 4, A met with B and C. A experiences symptoms a few hours into day 6. With a decentralised approach, you really need the certainty of a test before uploading anything, to stop Sybil-like attacks. Most probably, A would be tested the following day (day 7), and get a result (while isolating at home) a few hours into day 8. Meanwhile, B and C are walking around in the population, on day 7 and 8 (days 3 and 4 of their own infection, the time they are likely to be most infectious). They get asked to return home, and probably experience symptoms the next morning. In the NHS approach, when A experiences symptoms on day 6, they report this via the app, and are told to get home and self-isolate. B and C might also be asked to isolate, based on their contact with A, at this time (as well as their general perceived risk from any other infected individuals they might have had distant contact with - the NHS app can consider multiple potential sources of infection to an individual, and aggregate the risk). This gets B and C at home and isolated on day 2 of their own infection, and has them at home during their 2 most infectious days. If neither gets symptoms, and others exposed to A don't develop symptoms either, the daily "how are you feeling" polling via the app for symptoms will be able to capture that based on lack of symptoms, there likely wasn't exposure, and the group can leave self-isolation. Testing then augments this for more certainty, but this model does appear to be quite novel and unique, and near impossible to do in the decentralised approach (especially some of the more complex parts like deciding how likely A is to spread the virus, based on A's own exposure to infected people in A's recent past, and the date A reports symptoms). When taking this into account, I'm not sure how a "testing-based" model can work, unless there's pre-emptive screening being done. I don't think many countries have the capacity to pre-emptively screen asymptomatic people with the regularity and coverage you'd need to benefit from reacting to test results. [1] https://eu.usatoday.com/story/news/nation/2020/04/17/covid-19-patients-most-contagious-1-2-days-before-symptoms/5143641002/ https://eu.usatoday.com/story/news/nation/2020/04/17/covid-1...
- Mvandenbergh 6y ago
  Consider also the scenario where A reports symptoms on day n. They have logged contacts with 10 people on day n-2, 10 people on day n-3, etc. With the NHSX design, you might warn back to n-6 but might be able to determine that the riskiest cohorts are n-3 onwards. Since symptom onset distribution can be modeled, if by day n+3, not a single person in the n-6 and n-5 cohorts have developed symptoms (and maybe you've managed to get a -ve PCR from two n-5s - the fact that you have the contact graph means that you can use that data to adjust your risk score for all the others) you've now got a really good case for letting the n-5 and n-6 cohorts out. These models will be refined over time as well with new data. The presence of a central system which has some of this information lets you extrapolate from limited information. If you have infinite PCR tests and can logistically get them to potentially infected people very quickly, get them back to a lab, and get them tested then maybe this doesn't matter but not even South Korea's capabilities are infinite and there is no way the UK would be able to test all those people rapidly.

g_p 6y ago

I posted this in reply to another comment, but yes this does appear to be the flaw of the Google/Apple approach. Assuming the incubation time is just over 5 days (seems a broad consensus here), and someone is at their most infectious in the day or 2 prior to this (not sure if this is yet broadly agreed, but some studies are suggesting this [1]) Imagine that person A is infected on day 1. On day 4, A met with B and C. A experiences symptoms a few hours into day 6. With a decentralised approach, you really need the certainty of a test before uploading anything, to stop Sybil-like attacks. Most probably, A would be tested the following day (day 7), and get a result (while isolating at home) a few hours into day 8. Meanwhile, B and C are walking around in the population, on day 7 and 8 (days 3 and 4 of their own infection, the time they are likely to be most infectious). They get asked to return home, and probably experience symptoms the next morning. In the NHS approach, when A experiences symptoms on day 6, they report this via the app, and are told to get home and self-isolate. B and C might also be asked to isolate, based on their contact with A, at this time (as well as their general perceived risk from any other infected individuals they might have had distant contact with - the NHS app can consider multiple potential sources of infection to an individual, and aggregate the risk). This gets B and C at home and isolated on day 2 of their own infection, and has them at home during their 2 most infectious days. If neither gets symptoms, and others exposed to A don't develop symptoms either, the daily "how are you feeling" polling via the app for symptoms will be able to capture that based on lack of symptoms, there likely wasn't exposure, and the group can leave self-isolation. Testing then augments this for more certainty, but this model does appear to be quite novel and unique, and near impossible to do in the decentralised approach (especially some of the more complex parts like deciding how likely A is to spread the virus, based on A's own exposure to infected people in A's recent past, and the date A reports symptoms). When taking this into account, I'm not sure how a "testing-based" model can work, unless there's pre-emptive screening being done. I don't think many countries have the capacity to pre-emptively screen asymptomatic people with the regularity and coverage you'd need to benefit from reacting to test results. [1] https://eu.usatoday.com/story/news/nation/2020/04/17/covid-19-patients-most-contagious-1-2-days-before-symptoms/5143641002/ https://eu.usatoday.com/story/news/nation/2020/04/17/covid-1...

Mvandenbergh 6y ago
Consider also the scenario where A reports symptoms on day n. They have logged contacts with 10 people on day n-2, 10 people on day n-3, etc. With the NHSX design, you might warn back to n-6 but might be able to determine that the riskiest cohorts are n-3 onwards. Since symptom onset distribution can be modeled, if by day n+3, not a single person in the n-6 and n-5 cohorts have developed symptoms (and maybe you've managed to get a -ve PCR from two n-5s - the fact that you have the contact graph means that you can use that data to adjust your risk score for all the others) you've now got a really good case for letting the n-5 and n-6 cohorts out. These models will be refined over time as well with new data. The presence of a central system which has some of this information lets you extrapolate from limited information. If you have infinite PCR tests and can logistically get them to potentially infected people very quickly, get them back to a lab, and get them tested then maybe this doesn't matter but not even South Korea's capabilities are infinite and there is no way the UK would be able to test all those people rapidly.