Your immune system has different "compartments." The mucosal immune compartment produces its own antibodies and its own immune response. The "jab" did not immunize the mucosal immune compartment, so people could and did catch and spread Covid - but they were protected from developing a serious infection because the virus was unable to spread in their body further. As it invades through your respiratory system, vaccinating the mucosal immune compartment means its stopped at the point of entry and does not spread to others or develop into even a mild case.