This is not true. Do you have a medical background or are you a scientist, or are you just repeating what you’ve heard? I am a trained scientist and my husband is as well and he also works in the field and has much experience reading and interpreting clinical trial and safety data.
In a normal clinical trial only a few patients are recruited each month. For the covid trials they recruited around 40,000 patients in a month. Vaccines stimulate your body’s immune response so you build up antibodies to the virus. One of the vaccines has to be stored at very low temperatures, By the time it enters body and is there for a few hours it has done it’s job and degrades and is removed by the system as waste. Most issues with vaccines are immediate or within a few weeks (which they had plenty of time and patients to study) or they are interactions with something else the patient is taking. Because of the way vaccines work they have little long term effect on anything but your immune system. All the MMR causing autism stuff was completely debunked years ago, that “scientist” faked all his data and more has come out completely dispelling the myth that vaccines cause autism.