According to some grammars and CMS...
"Interjections and vocative should be separated from the rest of the sentence by commas".
The vocative case  is used to indicate direct address (i.e., to show when you are talking to somebody or something directly). Nouns in the vocative case are set off using commas. CMS 6.38
As direct address (i.e. vocative case), the parenthetical element should be set off using commas (CMS 6.30). An absolute phrase is always treated as a parenthetical element, as is an interjection.
That is a very classical way to hello the world.
But, languages change. I am not sure, but today, I feel that it is very uncommon helloing someone with an interjection, providing a small pause and clarifying who is helloed. I have an impression that usually people use hello as a ("kind of") verb, not an interjection.
Question: Is that just a personal (bad) impression or some linguistic has already studied that phenomenon?
-  - Nelson, Gerald C., Director Survey of English Usage Sidney Greenbaum, and Sidney Greenbaum. An Introduction to English Grammar. Routledge, 2013.
-  - Advanced Grammar in Use with Answers: A Self-Study Reference and Practice Book for Advanced Learners of English. 3 edition. Cambridge University Press, 2013.
- CMS - Chicago Manual of Style