Swordmaker, I’m going out on a limb here. I just ran across this research paper. https://www.sciencedirect.com/science/article/pii/S1521661620306513 You might want to give NAC a try together with Vitamin D. https://www.jomarlabs.com/products/n-acetyl-l-cysteine
I have no idea whether it will help or not. If you try it, please ping me back with your results. Disclaimer: I am not a medical professional. Always consult your doctor.
Thanks. On both already. It’s the sudden drop that really bothers me. I’ve had good lungs and a high, standard 97-98% level all my life. For it to drop to this level just after this low-level, mild illness is very strange, especially as I don’t feel short of breath. Odd. My doctors are at a loss too. . . Includes my cardiologist.