Vaccine immunity/protection doesn't kick in until the body has a chance to train the adaptive immune system. "Days" after vaccination is well within the range post vaccination that your body is still developing it's adaptive immunity.

The reality is that 99% of COVID deaths are in unvaccinated people, and the vaccinated people dying are high risk immunocompromised people. I'll take my chances of a minor allergic reaction instead of lifelong disability because I'm too much of a soft little bitch to get poked by a needle.