Why are so many people catching covid from the vaccines, if these are MRNA vaccines not containing live or even dead virus, from which is was said to be (and in theory should be) impossible to contract covid from?
You're hearing about people catching covid after vaccination, but there's no way they can catch it from the vaccine itself. Here are a few possibilities:
1. They caught covid BEFORE they got the vaccine. It takes an average of 5 days for symptoms to appear, but can be as much as 2-3 weeks later. During that interval they got the vaccine, and mistakenly thought that the vaccine caused covid, but they really were already infected before they got the vaccine.
2. They caught covid DURING the time they went for the vaccine. The most vulnerable people have been isolating at home, and when they go for the vaccine they may come in contact with people who are infected - for example, during the ride in a car or bus on the way to the vaccination place, or while sitting with other people who are waiting for the injection, or who are resting briefly after getting it.
3. They caught covid in the days AFTER the vaccine. The vaccine is not immediately effective. It takes a couple of weeks to start building up immunity, and only after the second injection does it become very strong. Even then, it's not 100%, so people need to continue to take precautions.