The virus has a high mutation rate relative to something like the flu virus. If everyone who could get the vaccine (e.g. not immunocompromised), the virus would not be able to mutate quick enough to transmit to others successfully. However, because not enough people are vaccinated, it is allowed to mutate in those it infects, hence new variants that may cause one to contract COVID, even if they were vaccinated. The vaccine that was released only teaches your immune system how to recognize a certain variant of COVID, but if the variant mutates enough, your body won't recognize it and can't fight it off, and you will contract COVID. Vaccines need herd immunity to be completely effective. It would likely work as intended and WOULD be a solution (as can be seen in countries where the vast majority of adults are vaccinated), but well.. here we are.