Search Ocrevus on the forum top left magnifying glass search symbol for a fuller answer. In short: it is precautionary to mitigate any possible reactions you may have. www.ocrevus.com (American site) explains it and I imagine the Aaron Boster videos do too. If you haven't watched them you will find them good. He had one on Ocrelizumab.

@dominics had a look at the search button but couldn't see the dosage which is what I'm looking for. I have since found it on a medical website 🙂 Thanks