Vaccines infect people with the very disease they supposedly prevent.
This conflates different vaccine types. Inactivated vaccines (like flu shots and polio) contain dead virus fragments, they cannot replicate or cause disease. mRNA vaccines don't contain virus at all; they're instructions for your own cells to make a protein. Live attenuated vaccines (like MMR, varicella, rotavirus) do contain weakened virus, and in extremely rare cases, immunocompromised individuals can develop attenuated disease, but this is orders of magnitude milder than wild-type infection and occurs in fewer than 1 in 1 million doses. Vaccine adverse event databases track these extremely rare events rigorously. The confusion arises because vaccines trigger symptoms like fever or soreness, these are signs of immune activation, not disease. A child vaccinated against chickenpox might develop a few chickenpox lesions (extremely rare), but not the thousands of lesions and risks of encephalitis or bacterial superinfection from wild varicella. The benefit-risk calculation is not even close: accepting the infinitesimal risk of vaccine reaction to avoid genuine disease with serious complications is one of medicine's clearest trade-offs.