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Comment by mytailorisrich | Hacker News Reader

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.

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?

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))

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.

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.

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.

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.

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.

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.

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?

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