Potential “Healthy Vaccinee Bias” in a Study of BNT162b2 Vaccine Against Covid

This NEJM correspondence critiques a previous[1] study that found a 90% drop in Covid-related mortality after the first booster shot compared to people who did not receive a booster shot.

It turns out that there is a similar drop in non Covid-related mortality, and the response from the authors of the original study confirms that.

So, either the first booster is a magic elixir that lowers mortality overall, or (much more likely) the people who took the first booster short were healthier than the people who did not take the first booster shot.

Observational studies, and especially those that compare self-selected groups are plagued with confounding variables and biases that are hard or impossible to control. This is not new, and the blind acceptance of these studies is, to me, concerning.

[1] discussed on HN at https://news.ycombinator.com/item?id=29522720

Edit:

This problem is not specific to this one study. As the authors of the original study note in their response:

> However, a strong, unexplained association between the use of the booster and lower mortality not related to Covid-19 remains. During the B.1.617.2 (delta) wave in the United States, similar associations were observed between the use of mRNA vaccines and lower mortality not related to Covid-191 and mortality from any cause.