The problem is some vaccines introduce something to your body (antibodies, for example) so if you then catch the disease/the vaccine doesn’t prevent infection, your system could be overloaded.
In any case, what they are injecting today fits no definition of “vaccine” as I’ve understood it (and they changed the official definition because a blind man could see that).
Real vaccines work. This vackseen doesn’t stop the spread. Hence, the difference in spelling.