At what stage do cold sores become contagious?

Until they dried : Until all dried and scabed, they considered contagious!

Herpes: Asymptomatic shedding is very real and one of the biggest reasons why it's possible for people with herpes to pass herpes virus on to a sexual partner without having any signs or symptoms. That's pretty much what asymptomatic means: you can't see or feel that the virus is present and active on the surface of the skin. Thus, the person with herpes is always contagious.