It is an anecdote, not data. Simple math: if Pfizer says their vaccine is 90% effective at preventing severe disease then 10% of people are at risk of getting severe disease. Not everybody catches COVID (masks, distancing, etc. do reduce the risk significantly, it seems), so say that’s another 90% reduction for each. That means that 1 in 1000 people you talk to will get severe COVID even when fully vaccinated.

Keep in mind that the person you are replying to is here to tell the tale. It is unclear what the vaccine did or did not do but if I was in their shoes I would certainly be glad that I wasn’t facing that infection unvaccinated. That could very well have been the difference between severe and deadly COVID. The effectiveness numbers have been always quoted as against severe illness but I would much prefer to focus on effectiveness against hospitalization and death. It seems that all three vaccines authorized/approved in the US are effective against these two outcomes, more so than merely protecting against a severe case.

Also keep in mind that what a random stranger describes on the internet as “severe” could mean a variety of things. I have had some severe colds. I haven’t had one since the pandemic started due to social distancing and masks. Don’t panic. Get your shots (booster included) and keep an eye on the data.