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/ )">https://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& #39;t control for the actual rates of vaccination either.

3/ Second, the Covid-19 data they use is of March-21, and don& #39;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)">https://www.medrxiv.org/content/1... 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".

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

fin.">https://www.forbes.com/sites/mad...