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I applaud the efforts to investigate whether smartphone technology can be a useful aid in the contact tracing process. Privacy aside, it seems to me that there

I applaud the efforts to investigate whether smartphone technology can be a useful aid in the contact tracing process. Privacy aside, it seems to me that there are two main issues that Bluetooth-beaconing-smartphones will struggle to solve: 1. Determining whether two smartphones "hearing" each other's beacon constitutes contact between the owners of those smartphones. (The blog post we're discussing brings up several examples, and the "orderly supermarket queue" of shoppers patiently waiting outside while observing a 2m distance could generate a massive number of false positives.) 2. Ensuring widespread adoption. If only 1/6th of the population is participating (which are the numbers Singapore is seeing so far) then the chance that an encounter between two people is captured by the app is 1/36th -- far too low to be of much use. There is also an upper limit to adoption because not everyone owns a smartphone or has a device which would be compatible with the contact tracing app.

mytailorisrich 6y ago
I wouldn't call being within 2m of a contaminated person a "false positive". I don't think these distances are absolutely safe and it's impossible to say exactly what interactions occured. The data from Bluetooth beacons are probably as good as is possible to get.
- kiwidrew 6y ago
  In an open space (such as a supermarket carpark) the Bluetooth LE range could easily be 10-20 meters. So one person standing in the queue could potentially be considered a contact of the 5+ people in front of them plus the 5+ people behind them. If you're not prepared to test all of those people (and potentially multiple times) then what action should the potential contacts take? Should they all isolate themselves for 14 days because they were in the same carpark as a confirmed case for a few minutes?
  - Normal_gaussian 6y ago
    presumably that is a pretty accurate sample of people likely to be infected by me. All the shoppers in the supermarket while I am (and shortly afterwards) are at risk of picking up the satsumas I fondled too much etc. (to be clear, I'm operating a 'touch means take' policy, but its clear through simple observation that most aren't capable of thinking through the rule (or even following it))
  - mytailorisrich 6y ago
    That's contact tracing and how to act on the information. As mentioned it's probably not possible to get better than that with a technological solution but it is much better than not using technology. It's also difficult to define what 'better' would be. Seems to me that all those people should indeed be careful and self-isolate.
  - jka 6y ago
    For what it's worth, I agree with you and believe your concerns are well-founded. I'd also note that the comment you're replying to didn't really address your two original points. Instead it stated some technically-correct but conversation-misdirecting facts.
  - mantap 6y ago
    2 meters is only sufficient for droplet transmission (e.g. breathing). Aerosol transmission (e.g. a sneeze or heavy cough) can travel much further than 2m. Granted these are for confined spaces and outdoors it will depend on factors such as the wind. But 10m sounds perfect. At the moment we are all isolating for months. So yes if 5% of the population needs to isolate because they were 10m away from someone who had it then so be it. I'm perfectly fine with that.
    - hsitz 6y ago
      You have aerosol and droplet mixed up. Aerosol is much finer particles than droplets. Sneeze would be droplet (plus aerosol), breathing would be aerosol, likely not droplet. From what I've read it's not clear that aerosol would be limited to 2m, e.g., if it were allowed to build up in enclosed space. But in any case it's all on a curve, the further away you get the less likely a person will be infected.
      - mantap 6y ago
        That's the opposite of what I heard, that breathing mostly produces droplets. But I wouldn't be surprised if it's wrong, it seems like this area of research is not well understood by public health professionals. Regardless of droplets or aerosols, sneezes definitely have a high velocity and travel further than 2m. And yes it's a curve so the risk depends on the minimal infective dose. If you can get infected by a small number of virus particles (which is apparently the case) then you need more distance.
  - jmull 6y ago
    Bluetooth beaconing can estimate the distance between two devices (and does better if, eg, given 10 seconds to estimate vs. 1 second). I assume a covid-19 contact tracing system would use this, among other things, to filter out contacts that don’t meet the established criteria for high enough risk of transmission.
  - jeremyjh 6y ago
    Indeed it cannot work without very widespread testing. We'd need the capacity to test millions of cases a day - maybe 20-30 million. Just testing all our front-line health care providers daily would require many millions of tests. There is currently no one in the United States developing a plan to test at even a small fraction of that scale. It is definitely possible, but would require an administration capable of doing something other holding press conferences and tweeting malice and misinformation. So, maybe next time?
arrrg 6y ago
False positives are ok, even a massive amount of false positives. Currently we all have to change our behaviour around everyone, all the time. Even if you are in New York that probably means there are “false positive” behaviour changes (behaviour changes for no reason) all the time – simply because we lost control of the situation and cannot currently contact-trace our way out of that situation. The idea behind these contact tracing apps is that they make only really sense once you have to outbreak well under control (through drastic measures). The goal is then to relax measures (probably not to abandon all of them – handshaking is probably dead and the recommendation will remain to stay away from other people in public and forget large gatherings for a long time, as well as a lot of travel …) and be much more targeted with who you impact through such contact tracing. Obviously they cannot replace proper contact tracing, but they can be a helpful tool in the tool-belt and increase the effectiveness of standard contact tracing and testing. If there are, say, 500 detected active cases in a country like Germany (which is probably around the limit of where contact tracing still works, even with many more people working contact tracing and much expanded testing capability) contact tracing apps can provide an additional signal and increase the speed of standard contact tracing approaches, especially since we know that much of the spread is pre-symptomatic. Even if this tool overestimates the infectious contacts someone had hundredfold and we assume a ridiculous R0 like 10 (so someone infects ten people on average but a hundred times that many people are actually detected by contact tracing – so 1,000 people altogether) that would still only place at most 500,000 people in quarantine (assuming an upper limit of 500 detected active cases that can be handled before you lose control again and have to place the whole country under lockdown). That’s a huge number but probably not as impactful at all as the current measures that are in place. (Obviously you need to put measures in place to make that possible: mandatory, automatic and paid sick leave for anyone who is told to stay at home for a week or symptom-end plus one week. Stuff like that. Some countries are probably better prepared for that than others.) Even this blunt instrument would be much better than the current situation.
- reitzensteinm 6y ago
  Yes, and the author's description of being notified for having been within 3 meters of someone with Covid as being "punished" with "false alarms" makes me want to tear my hair out. If contact tracing were enabled and everyone at any risk self quarantined this nightmare would soon be over. Instead people will justify why they're low risk, interact with others and continue the spread.

mytailorisrich 6y ago

I wouldn't call being within 2m of a contaminated person a "false positive". I don't think these distances are absolutely safe and it's impossible to say exactly what interactions occured. The data from Bluetooth beacons are probably as good as is possible to get.

kiwidrew 6y ago
In an open space (such as a supermarket carpark) the Bluetooth LE range could easily be 10-20 meters. So one person standing in the queue could potentially be considered a contact of the 5+ people in front of them plus the 5+ people behind them. If you're not prepared to test all of those people (and potentially multiple times) then what action should the potential contacts take? Should they all isolate themselves for 14 days because they were in the same carpark as a confirmed case for a few minutes?
- Normal_gaussian 6y ago
  presumably that is a pretty accurate sample of people likely to be infected by me. All the shoppers in the supermarket while I am (and shortly afterwards) are at risk of picking up the satsumas I fondled too much etc. (to be clear, I'm operating a 'touch means take' policy, but its clear through simple observation that most aren't capable of thinking through the rule (or even following it))
- mytailorisrich 6y ago
  That's contact tracing and how to act on the information. As mentioned it's probably not possible to get better than that with a technological solution but it is much better than not using technology. It's also difficult to define what 'better' would be. Seems to me that all those people should indeed be careful and self-isolate.
- jka 6y ago
  For what it's worth, I agree with you and believe your concerns are well-founded. I'd also note that the comment you're replying to didn't really address your two original points. Instead it stated some technically-correct but conversation-misdirecting facts.
- mantap 6y ago
  2 meters is only sufficient for droplet transmission (e.g. breathing). Aerosol transmission (e.g. a sneeze or heavy cough) can travel much further than 2m. Granted these are for confined spaces and outdoors it will depend on factors such as the wind. But 10m sounds perfect. At the moment we are all isolating for months. So yes if 5% of the population needs to isolate because they were 10m away from someone who had it then so be it. I'm perfectly fine with that.
  - hsitz 6y ago
    You have aerosol and droplet mixed up. Aerosol is much finer particles than droplets. Sneeze would be droplet (plus aerosol), breathing would be aerosol, likely not droplet. From what I've read it's not clear that aerosol would be limited to 2m, e.g., if it were allowed to build up in enclosed space. But in any case it's all on a curve, the further away you get the less likely a person will be infected.
    - mantap 6y ago
      That's the opposite of what I heard, that breathing mostly produces droplets. But I wouldn't be surprised if it's wrong, it seems like this area of research is not well understood by public health professionals. Regardless of droplets or aerosols, sneezes definitely have a high velocity and travel further than 2m. And yes it's a curve so the risk depends on the minimal infective dose. If you can get infected by a small number of virus particles (which is apparently the case) then you need more distance.
- jmull 6y ago
  Bluetooth beaconing can estimate the distance between two devices (and does better if, eg, given 10 seconds to estimate vs. 1 second). I assume a covid-19 contact tracing system would use this, among other things, to filter out contacts that don’t meet the established criteria for high enough risk of transmission.
- jeremyjh 6y ago
  Indeed it cannot work without very widespread testing. We'd need the capacity to test millions of cases a day - maybe 20-30 million. Just testing all our front-line health care providers daily would require many millions of tests. There is currently no one in the United States developing a plan to test at even a small fraction of that scale. It is definitely possible, but would require an administration capable of doing something other holding press conferences and tweeting malice and misinformation. So, maybe next time?

arrrg 6y ago

False positives are ok, even a massive amount of false positives. Currently we all have to change our behaviour around everyone, all the time. Even if you are in New York that probably means there are “false positive” behaviour changes (behaviour changes for no reason) all the time – simply because we lost control of the situation and cannot currently contact-trace our way out of that situation. The idea behind these contact tracing apps is that they make only really sense once you have to outbreak well under control (through drastic measures). The goal is then to relax measures (probably not to abandon all of them – handshaking is probably dead and the recommendation will remain to stay away from other people in public and forget large gatherings for a long time, as well as a lot of travel …) and be much more targeted with who you impact through such contact tracing. Obviously they cannot replace proper contact tracing, but they can be a helpful tool in the tool-belt and increase the effectiveness of standard contact tracing and testing. If there are, say, 500 detected active cases in a country like Germany (which is probably around the limit of where contact tracing still works, even with many more people working contact tracing and much expanded testing capability) contact tracing apps can provide an additional signal and increase the speed of standard contact tracing approaches, especially since we know that much of the spread is pre-symptomatic. Even if this tool overestimates the infectious contacts someone had hundredfold and we assume a ridiculous R0 like 10 (so someone infects ten people on average but a hundred times that many people are actually detected by contact tracing – so 1,000 people altogether) that would still only place at most 500,000 people in quarantine (assuming an upper limit of 500 detected active cases that can be handled before you lose control again and have to place the whole country under lockdown). That’s a huge number but probably not as impactful at all as the current measures that are in place. (Obviously you need to put measures in place to make that possible: mandatory, automatic and paid sick leave for anyone who is told to stay at home for a week or symptom-end plus one week. Stuff like that. Some countries are probably better prepared for that than others.) Even this blunt instrument would be much better than the current situation.

reitzensteinm 6y ago
Yes, and the author's description of being notified for having been within 3 meters of someone with Covid as being "punished" with "false alarms" makes me want to tear my hair out. If contact tracing were enabled and everyone at any risk self quarantined this nightmare would soon be over. Instead people will justify why they're low risk, interact with others and continue the spread.