The North Vietnamese were able to keep supplies moving down the Ho Chi Minh Trail throughout the war. From the link:
"From the air the Ho Chi Minh Trail was impossible to identify and although the United States Air Force tried to destroy this vital supply line by heavy bombing, they were unable to stop the constant flow of men and supplies. The main danger to the people who travelled on the Ho Chi Minh Trail was not American bombs but diseases like malaria. In the early days, as many as 10 per cent of the porters travelling down the trail died of disease."
Ho Chi Minh Trial
