Uncertain effectiveness of Covid-19 vaccination

My take on what this study is trying to show:

Imagine giving two doses of a strong laxative to the general population. Everyone is exposed to covid.

Statistically you could show that those that have been twice laxidated did better against covid. But, what this ignores is that the laxative is a fitness test of sorts. If a 70 year old survives two doses of a strong laxative, and did not die, they're not from the weakest subpopulation.

Study:

https://www.researchgate.net/publication/356756711_Latest_st...