All these observational studies are heavily criticised for not being a placebo controlled randomized double blind however. Some critics are honest enough to admit that their main point is that studies like this could led people to stop using masks and wanting vaccination.
This is the German observational study:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7551780/
This is a walkthrough of a Spanish study by DrBeen Medical Lectures:
All these observational studies are heavily criticised for not being a placebo controlled randomized double blind however. Some critics are honest enough to admit that their main point is that studies like this could led people to stop using masks and wanting vaccination.
This is the German observational study:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7551780/
This is a walkthrough of a Spanish study by DrBeen Medical Lectures:
https://youtu.be/IpFdbu_AMI8?t=180
(post is archived)