Your question is a Great question. Unfortunately I do not have or have I been able to find an answer.
You know, I don’t believe a darn word about the period of contagiousness. I think this is simply an excuse to keep the public calm or unaware and something the CDC & king putt can use before they tell us, “We just didn’t know,” or “It’s mutated into something stronger... and earlier period of contagiousness,” etc.