With the vaccine, you're a created transmitter in addition to naturally occurring transmitters. Like I mentioned before. Imagine if when they rolled out the polio vaccine that everyone they vaccinated immediately gave everyone else polio. The question then becomes, did you really give people an immunity, or did you infect everyone in a way so they only had mild symptoms but they're still contagious? This has huge implications on societal immunity.
For example with a flu vaccine, it provides immunity through an immune response which results in immune memory. They are not contagious though. So the benefit is the vaccine creates these immune barriers of people. And if you have enough immune barrier people you can halt that specific transmission chain. AKA a vaccinated kid comes in contact with a kid at school that has the flu, he can't bring it home and give it to his non-vaccinated mom because he has vaccinated immunity, and as a result, is not a carrier or contagious. Now let's flip the script, if he gets a vaccination that makes him a carrier and contagious, he will not only give it to mom but every kid at school, and everyone else he comes in contact with. The barrier becomes the spreader. When looking at the reason vaccines work, a vaccine that makes someone contagious is complete lunacy. The only way to get around that is to mandate that everyone get the vaccine in the shortest amount of time possible.