The short answer is that its not. In general, its best to wait until after the scabs and sores completely disappear before you kiss someone or engage in oral sex. This is because the herpes virus can continue shedding in the late stages of a cold sore healing, even if theres no viral fluid present.
Can you spread cold sores even if you dont have one?
Cold sores are most contagious when you have oozing blisters because the virus easily spreads through contact with infected body fluids. But you can spread the virus even if you dont have blisters. Many people who are infected with the virus that causes cold sores never develop signs and symptoms.
Are cold sores contagious forever?
Many people believe cold sores are no longer contagious once theyve scabbed over, but this isnt true. You are most contagious for two weeks after the cold sore initially appears, but because the virus lives in the body forever, it is always transmittable to others.