That BMJ Effectiveness of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines ... paper is not a 'preliminary study'. It's an International Committee of Medical Journal Editors peer reviewed case-control study.
But, you do you.
I would also read the limitations section in that study where they say their findings should be treated with caution. The main issue is that the study does not take into account the habits of each individual post vaccination, so whether the vaccine itself is responsible or such things like how the individual isolated. The WHO data also takes into account generalised effectiveness over a number of different cultures, communities and age groups. Again taking studies out of context with no medical background can lead you to make assumptions that are not valid. The WHO information is dumbed down for the average public to follow and is based on general information.