There's always a balancing act between vaccine safety and efficacy. The smallpox vaccine mimics a natural infection, but it's extremely dangerous because people who get vaccinated can actually spread real smallpox to others for a period of time, so they have to stay isolated.

Pre-Covid coronavirus vaccines tested in animals induced antibody-dependent enhancement and were thus deemed unsafe for human trials. A safe, effective Covid vaccine was always a long shot. Maybe one day we'll get one, but maybe we won't. Thankfully, there are multiple other drugs that are effective at preventing and treating Covid.