It is in regards to an introductory phrase and when you need a comma after one and when you don't. For instance, do you need a comma after the words "in addition" when it starts a sentence?
Or what about "This week"? Some of the sites I have looked at say that you only need a comma if the prepositional phase is three words or longer, but I'm not quite sure if those two examples are prepositional phrases or introductory phrases.
I think you probably do need a comma after "In addition." Here's why:
There's a distinct pause in "In addition" at the beginning of a sentence. The comma signals that pause to the reader.
Unfortunately, you can read different interpretations of that rule. I usually advise no comma after a prepositional phrase unless it's five or more words long. However, I think you need the comma here. (Go to the Grammar Doctor and click on punctuation> comma rules 1 for more on introductory commas.
The Gregg Reference Manual says you can omit the comma after a short introductory prepositional phrase if 1) it doesn't have a verb form, 2) is not transitional, and 3) the sentence is still clear.