In the sentence "You, too.", is the comma acceptable?

Question

I tend to think it is, as "too" serves the role of a referential phrase, repeating the verb-phrase of a preceding sentence, and "you" acts simply as a subject pronoun. But, I've heard an argument to the contrary.

Answer 1

In fiction, you might get away with an occasional sentence fragment, but in academic writing, you need complete sentences. "You, too" is a fragment.

Answer 2

The comma is necessary. I wouldn't think of "You, too" in strict grammatical terms - it's really just a colloquialism that serves to echo a sentiment without reiterating it completely.