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IVIG treatments for all sorts of diagnoses generally take place every 4 weeks or else more frequently, depending on the dosage and manner. For example, we have some patients who receive their IVIG every 2 weeks -- but then they receive half the dosis that they previously got every 4 weeks. Look it up on the Internet and see, e.g., at http://www.usbioservices.com/specialties/ivig/treatments
And the reason for this is that IVIG is a blood product, and as the body's own blood is renewed continuously, whatever IVIG a patient receives on Day One is diluted away each day after that. Studies show that half of the dose received on Day One is gone from the body by Week Three or Four, and the remaining half disappears very rapidly after that. (It's not a straight-line pace of decay.) Therefore, in order for sufficient IVIG to be always present in the bloodstream -- namely, at least a half-strength -- patients must receive additional amounts by Week Three or Four.
In sum, getting IVIG every 5 or 6 weeks might potentially be justified if the initial dose is so powerful that the patient can do with just a tiny fraction of the original amount by Week Five or Six, but I don't know of any justification for giving it every 3 months -- other than for the insurance company to save money on Levi by letting him run on "empty" for a month-and-a-half.
But don't take my word for it. Feel free to have Levi's doctor consult with an immunologist or else with Dr. Kirk Druey at NIH, firstname.lastname@example.org He's the world's leading authority on SCLS, and in 2015 he published a co-authored article on SCLS in six children, see http://pediatrics.aappublications.org/content/135/3/e730
The authors concluded that "Four of our 6 patients were treated with IV or SCIG [subcutaneous immunoglobulins], and none had a further severe episode while on this treatment, suggesting that immunoglobulin prophylaxis (1–2 g/kg per month) is effective therapy for SCLS in children." [The other two children could not tolerate the indicated dosis all at once, so they were switched to receiving an equivalent amount subcutaneously, over many more days, in amounts carefully designed to provide the same protection as 1-2 g/kg per month.]