link to original reddit post by /u/Perleflamme
They tried to train AI to predict various patient's state, but ended up discovering they couldn't use most of the available data among more than 2200 papers: only 6 papers could show data with low risk of bias.
https://venturebeat.com/2021/03/23/major-flaws-found-in-machine-learning-for-covid-19-diagnosis/
The actual Nature paper: https://www.nature.com/articles/s42256-021-00307-0