it's a spike protein, covid and vaccine both have it. The spike is literally a spike that attached to your body. If the vaccine is already all attached in your body covid has less of a chance to attach itself to you. Of course the vaccine also causes problems such as scarring blood vessel wals or the inside of your heart, hence why it causes clots and inflammation of the heart. Your body having immunity, with antibodies constantly killing the virus, is better of course.