As yet and Then

As yet adverb - Used in negative statement to describe a situation that has existed up to this point or up to the present time.

Then and as yet are semantically related. In some cases you can use "Then" instead an adverb phrase "As yet".

Then adverb - In addition to what has been said.

As yet and then are semantically related. Sometimes you can use "As yet" instead an adverb "Then".