Notably, the second shot actually has been observed to reduce immunity in patients who have recovered from COVID and who have acquired natural immunity.
The mRNA jab is in itself a foreign invader that will provoke an immune response. A normal vaccine introduces a weakened or dead version of a virus into the body, provoking an immune response against it. But the mRNA jab introduces instead introduces a program to generate an immune response against something other than itself. So it has to first suppress the body's immune response against the program so that the program can be allowed to run.
The problem is that this immune response suppression does not only suppress the immune response against the mRNA jab program, but that suppression spills over against other agents. Thus, the immune response against the virus itself (one it breaks through the protein coat and reveals itself to the body) is compromised.