A few headlines popping up in the UK at the moment on "Supermarkets most common #COVID19 exposure location in England, data shows".

Yes, the data does show this, but don't be misled by incorrect interpretation!

What is this data?

It's the latest @PHE_uk surveillance report, which looks at 34,328 #COVID19 cases with a common exposure with at least 1 other case, over the period 9th - 15th November


Here, "common exposure" means any location/event which at least 2 #COVID19 cases attended in the 2-7 days before testing positive.

If you rank these common exposures, you find supermarkets are the most often reported (18.3%).

But place of exposure ≠ place of infection !

What the data is telling us is that most people visited supermarkets before testing positive for #COVID19, not that they were infected there. Reaching that extra conclusion requires further contact-tracing investigation.

But still, why are supermarkets most common then?

Think about it this way: how many people visited supermarkets but did not then test positive for #COVID19?

In short, supermarkets ranking number 1 is likely just explained by the fact that everyone goes to supermarkets.

Also, remember the time period of the data: 9-15th November.

Lockdown started on the 5th. This reduces #COVID19 exposure risk in closed places (pubs, gyms etc...), while artificially inflating the relative risk in places which do remain open (supermarkets!)

Tldr; please don't be misled by the headlines, place of exposure ≠ place of infection, further investigation is necessary to determine this!

Blindly labeling supermarkets now as "high risk #COVID19 transmission settings" is meaningless.


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More from @QuentinLclrc

27 Oct
Currently a lot of discussions around #COVID19 superspreading events, so it's time for a thread to add some context and reply to common comments about this...
"*random setting* is not in the database! So it must safe!"

That's a risky conclusion to make. Not detecting transmission ≠ transmission didn't happen! There are a lot of biases that make it hard to identify some settings. We also have to remember that...
... many setting have been closed / reduced visitors these past few months. Hard for transmission to happen in a setting when no-one's there! And, just based on that, risky to assume there won't be any transmission when people come back.
Read 9 tweets

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