Ha ha.... Sometimes it is good to under-rate ourselves so that those who boasted a lot will be in real surprise.
WE all know very well that North Vietnamese MIG-21 retreated to South China when US fighter jets went MIG hunting. They only came back to North Vietnam after American fighter jets flying back to base.
From Global Security.org:-
On 2 January 1967 US Air Force fighters set an intentional trap for the PAVN Air Force, destroying almost half of the PAVN's operational MiG-21 inventory. Hanoi once again ordered a stand-down in order for its pilots to receive better training. VNAF MiGs failed in their primary mission to stop US air attacks over North Vietnam during OPERATION ROLLING THUNDER. In fact, the VPAF fighter force sometimes retreated to China and stood down from combat operations due to heavy losses suffered at the hands of American fighter crews.