Quite a few public health professionals have stated that asymptomatic individuals, while infectious, are a very low risk vector. They don't cough or sneeze and the primary transmission rate is through droplets. I can try finding a few sources.
>>yitian+(OP)
Yet empirically there are symptomatic individuals showing up all the time, with no evident vector. So asymptomatic folk are assumed to be contagious, and not just a little.