You may have come across the "BCG hypothesis" lately - strong correlation between universal #BCG vaccination & protection against #Covid19. See attached abstract as an example. These studies received much media attention.

But wait, how sound are these results?

Thread 1/

2/ First, the study relies on the "BCG atlas" ( http://www.bcgatlas.org/ ) to make these claims. But the BCG atlas only documents the "vaccination policy" not the actual level of vaccinations. Further, the study also doesn't control for the actual rates of vaccination either.

3/ Second, the Covid-19 data they use is of March-21, and don't control for date of onset. Whether BCG helps is a moot point for a country that has not seen onset of community transmission. Comparison should be done after a fixed number of days of onset of community transmission.

4/ This paper ( https://www.medrxiv.org/content/10.1101/2020.04.09.20056903v1) does exactly that and the report that entire correlation vanishes upon controlling for date of onset.

5/5 @paimadhu, an epidemiologist, who put together the BCG atlas, discusses multiple issues with these studies as well as the "uncritical media attention, even before peer-review".

https://www.forbes.com/sites/madhukarpai/2020/04/12/bcg-against-coronavirus-less-hype-and-more-evidence-please/#7c08627a6b4f

fin.