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Comment by calsy | Hacker News Reader

How are infections measured in seconds?

Up until about 2 months ago contact tracing in Australia was very effective. Contact tracing teams were able to trace who and where each person got infected. They were also notifying/testing/isolating first and sometimes secondary downstream contacts (up to 400 people in the case of one country town outbreak).

One of the infections was traced to a department store and the interaction between the two people was caught on cctv. They walked past each other and didn't talk. It was seconds.

It’s not like you can see the viral particles jumping over in that exact moment. I’m sure there is more to the story, but let’s not try and claim that this is incontrovertible scientific proof that infection happens “in seconds”.

If:

* A vast majority of infections can be traced

* You have an infection whose only traceable contact is for a few seconds in a store...

It seems pretty likely that the infection happened in the moments surrounding those few seconds. Especially when we have multiple circumstances like this. Especially when it agrees with our underlying understanding of the germ theory of disease.

Extended contact makes infection much more likely. But people who are sick shed live virus, and there's not a safe level of exposure of live virus where infection is impossible.

Not necessarily. I'd be interested to know if the virus samples from those two people were sequenced in order to prove transmission from one to the other. Absent that evidence there could be other explanations such as : transmission was from some third person not known to the contact tracers.

> transmission was from some third person not known to the contact tracers.

This becomes an unlikely explanation once

> > * A vast majority of infections can be traced

and

> > Especially when we have multiple circumstances like this.

The probability of an alternate explanation becomes exceptionally unlikely. It's further reduced by our general scientific understanding that:

> > But people who are sick shed live virus, and there's not a safe level of exposure of live virus where infection is impossible.

Infection from a momentary contact is fairly unlikely. But when you have a massive number of momentary contacts in the populace, it happening many times becomes a near certainty.

E.g., this:

> Several individuals in Australia were also infected with the Kappa variant through lingering virus aerosol particles in the hallway of a quarantine hotel in May. Though the individuals had no direct contact with each other they opened the doors to their hotel rooms within 30 minutes of each other and tested positive for the same strain.

No complete sequencing, but the same variant... spread several times apparently by the same mechanism of indirect contact, several individuals infected. In a quarantine hotel, where there's not many alternate explanations available.

Or this one, confirmed by sequencing:

> A similar event occurred earlier in April where two families quarantining in rooms next to one another were found to share the same viral sequence, after briefly opening their doors 30 minutes apart.

https://www.forbes.com/sites/williamhaseltine/2021/06/28/inf...

In Australia earlier this year when infections were happening the rate of single-digits per day, they could trace the source of an infection to a fleeting encounter of two people walking past each other in a shop.

Source: https://www.abc.net.au/news/2021-06-22/covid19-cctv-footage-...

I haven't followed Australia's progress with COVID-19, so I hope these questions aren't too obvious or bothersome:

Were these single-digit infections all symptomatic, or were they detecting asymptomatic infections too? What is the false positive and false negative rate of the type of testing Australia is doing, and how many people were being tested during this time?

If some of the infections were asymptomatic, is it possible that some of them are false positives, and the fleeting encounter was also a false positive (either a false positive test, or that wasn't the true source of the infection) just due to random chance?

I have not seen widespread data on symptomatic vs unsymptomatic cases however during the most recent NSW surge around 5-7% of new infections ended up presenting to hospital at some stage. Testing was widespread and likely highly compliant - daily testing stats were available, NSW at one point was having 180k tests per day (PCR). People were notified of sites of exposure (or via checkin apps) and asked to test. Given it was PCR, false positives were as you would expect quite low but there were at least 2-3 false positives I recall being discussed in the media.

In answer to your final question - no - they were PCR confirmed cases. There was also a high degree of sequencing occurring to enable clarity on lineage transmission

Positives would be re-tested to rule out false positives.

In that period 30k-60k people where being tested in NSW. https://www.covid19data.com.au/nsw

I think the person meant that it only took seconds of total contact time to cause an infection. Even if the infection took days to develop.

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