Thread by @akm5376

Sometime early last year, an arbitrary >15 minute limit was placed on classifying close contacts for tracing #Covid_19 transmission.
As @jmcrookston puts it, no one knows for sure why 15 minutes and believe me, if he is not sure, no one is. No one has looked harder. A 🧵 1/7

Gradually over the year though, evidence started emerging that the virus does not wear a watch, i.e., 15 minutes is not a magical barrier.
Busting the 15-minute myth further: 2/7

cdc.gov/mmwr/volumes/6… Outbreak in a prison. Correctional officer infected with ~17 minutes of cumulative exposure, not continuos, and he was wearing masks during all interactions 3/7

Revised guidance from CDC. "Individual who has had close contact (within 6 feet for a total of 15 minutes or more)" Now 15 mins or more, cumulative, over 24 hours.
cdc.gov/coronavirus/20… 4/7

Two infections, from one person, seated over 4 meters away. One infection in ~5 minutes spent in same room 5/7
jkms.org/DOIx.php?id=10…

"National Football League observed SARS-CoV-2 transmission after <15 minutes of cumulative interaction, leading to a revised definition of a high-risk contact that evaluated mask use and ventilation in addition to duration and proximity" 6/7
cdc.gov/mmwr/volumes/7…

So 15 minutes is not magical, neither is 2/1.5/1 m
There is no absolute guarantee of safety.
To reduce your chances of getting infected, reduce time in contact, #MaskUp, keep as much distance as possible, avoid crowds and indoors, #ventilate #FreshAir #UseAirbornePrecautions 7/7

And I could not think of a better way to put the complexity of defining close contact than quoting this thread from Prof. Corsi (@CorsIAQ)

https://twitter.com/CorsIAQ/status/1326744878609436672

One more instance of under 15 (minutes)

https://twitter.com/Don_Milton/status/1356454024992669696