Hello Walt and welcome back! I have emailed you a few times in the past couple of years without success, because I wanted to make sure you were okay from SCLS, and perhaps I didn't hear back because you changed your email address? Anyway, so sorry to hear about your back problems! Feel free to write to me directly at my old email address, or else through this new platform, which allows person-to-person, private communication (see the "Message" icon.)
To your question, you've got nothing to fear from switching IVIG brands. There can always be a bad batch once in a million, because after all it's a plasma product and not an artificial, lab-manufactured compound, but the product is generally obtained in a similar manner with stringent quality controls no matter the brand. I have been given Privigen, Gammagard and Gamunex-C over the years, and I've had no adjustment problems.
The reason why different health systems, hospitals and clinics use different brands is because they shop around and make their own deals with different manufacturers. And I think the most common reason why they sometimes "run out" of a brand is probably because they are wanting to impress upon their current provider that they are willing to switch brands if they can get better pricing. I haven't heard of any "shortage" in the USA.
Wishing you better health, Arturo