It's not a new idea, but the implications if it goes wrong are severe. Imagine it mutates (it happens, especially if it's super infectious and thus being exposed to MANY environments and gets to live millions of years per day (time * infected humans)). Imagine it works differently in a small percentage of people, it's a lot harder to control when it's distributing itself.. With vaccines, at least, administration can be stopped or paused, if cause for concern arose.