What does the date on my phone's COVID exposure thingy mean?
September 30, 2021 7:07 AM   Subscribe

My iPhone told me Sept. 29 that I had possibly been exposed to COVID-19 some time in the previous two weeks. When I look at the relevant screen in the Exposure Notifications system, it says "You may have been exposed to COVID-19 / September 21, 2021." What's that date for? Did it detect it eight days ago and not get around to notifying me?

I'm vaccinated, I don't have any symptoms, and I've been tested for COVID one million times in the past month anyway. I'm just curious how this works.
posted by The corpse in the library to Computers & Internet (13 answers total)
Best answer: If it works like our state Covid app, someone had to first themselves get Covid, get symtoms, get tested and test positive, and then log into the app to say they were positive. The app then miscellaneously alerts all other phones with the app that were within a set exposure distance (I think "close proximity" for at least 15 minutes) that you may have been exposed. It generates some sort of blind bluetooth code or something (sure someone can actually explain this) so it's totally anonymous but you do get that alert. The code would be from a specific time/date, so yes it seems to be saying 9/21 was the date of exposure, but it notified you as soon as the other person's positive result was entered into the app.
posted by DoubleLune at 7:12 AM on September 30, 2021 [18 favorites]

Maybe because the person who you were close to didn't receive a positive test result until Sept. 29? The phone keeps track of everyone you're close to, and then when someone tests positive it looks back at who they were close to during the period when they were contagious. It notifies those people.
posted by jkent at 7:13 AM on September 30, 2021 [2 favorites]

Did it detect it eight days ago and not get around to notifying me?

I would guess that the system was only recently aware of a transmission at that site (not sure if this system is about transmission or exposure), that happened eight days ago. The people in question might've been diagnosed just recently.
posted by pompomtom at 7:14 AM on September 30, 2021

Best answer: No. What happened likely was:
a) on the 21st, you were in close proximity to some number of people, including Joe.
b) on the 27th or something, Joe got a COVID test for whatever reason
c) on the 29th, the results of that COVID test came back positive, and were reported to the exposure tracker.
d) the exposure tracker then found everyone who had been in close proximity to Joe In the last 2 weeks, and notified them.
posted by brainmouse at 7:14 AM on September 30, 2021 [9 favorites]

The way I read this article: your phone keeps track of where you’ve been for the last 14 days. Other users’ phones do the same. If someone tests positive, their phone can send alerts to the people it was close to in the last 14 days.
posted by Night_owl at 7:14 AM on September 30, 2021

Response by poster: I thought that it wouldn’t tell us when the exposure was, to protect the privacy of the person who tested positive.
posted by The corpse in the library at 7:21 AM on September 30, 2021

You need to know the exposure date to know when to get tested yourself (anytime is a good time to do it at this point) and if unvaccinated (perhaps you have young kids who presumably aren’t vaccinated or have their own phones?) how long to quarantine for as well.
posted by raccoon409 at 7:29 AM on September 30, 2021 [2 favorites]

Best answer: And having worked in contact tracing, the timeline probably went
sept 21-you we’re around this person
Sept 23- they started having symptoms
Sept 26- they were able to get a Covid test
Sept 28-29they got their results back but because of bureaucracy they had to call a few different numbers to get the pin to enter to confirm they are Covid positive in the app

(Remember that people are considered contagious for 48 hours before they start having symptoms or test positive)
posted by raccoon409 at 7:32 AM on September 30, 2021 [2 favorites]

Response by poster: Huh! Based on that, it would be easy for me to figure out who the person is. Interesting.
posted by The corpse in the library at 7:42 AM on September 30, 2021

It's mostly designed that way so the authorities (people running the system) don't know who you are, not that you can't possibly figure out who might have exposed you.

But if you ate in a restaurant that day, it could have been anyone there, or someone you ride the bus with. That's the value of the system, letting you know about exposures from people who aren't part of your institutions (work, childcare, church, etc).
posted by flimflam at 8:12 AM on September 30, 2021 [5 favorites]

Best answer: FWIW, in Massachusetts the notification date given can be +/- 1 one day and is based on UTC without timezone correction. I know this because I was notified of an exposure on a date when I didn't leave my house. I corresponded with the Mass Notify office, who in turn corresponded with Apple and came back with this information. I expect it's the same around the country.
posted by Winnie the Proust at 8:17 AM on September 30, 2021 [12 favorites]

A little more info about the exposure date given by the Exposure Notification system. Silly me, I thought the purpose of the date was to help me figure out where/when/how I got exposed. But that's not it. The purpose of the date is to set your quarantine & testing schedule, as described here. Don't bother trying to use the date for anything but that.
posted by Winnie the Proust at 8:53 AM on September 30, 2021 [5 favorites]

Note these exposure apps use a two part system that doesn't actually identify any individuals. Joe's phone (and your phone) create random IDs that aren't linked to your names at all. The first part of the system is basically just a running list your phone keeps of all the IDs it's been near. Any phone using the system shares its current random ID with all the other phones that are nearby, and those phones log the IDs along with timestamps. These IDs also change periodically to make it harder to identify any individual than it would be if they always have the same ID. The next time you see "Joe" he may have a new ID, so your phone will log that new ID instead of the old one, but your phone won't know it's "Joe" even though you might.

If "Joe" tests positive, he logs back in and pushes a button that notifies the system of the ID(s) his phone has used recently, and then that ID (or those IDs) get published in a list. The second part of the system kicks in after publication of that list: your phone periodically downloads the latest version of the list and looks for any IDs that are in its own log. In this case the ID "Joe" had around September 21 was in both your own phone's list and the new download, so your phone alerts you of the potential exposure.

I know if I were in a social setting and subsequently tested positive, I'd personally reach out to my friends to let them know and not rely on a notification system. But that's just me.
posted by fedward at 12:28 PM on September 30, 2021 [3 favorites]

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