Indeed, I have used D3 (10,000 IU Daily), Iodine 50mg daily with co-supps, Vitamin C & K2 for years to no avail. I have also used SSKI 800mg that did help somewhat, at least reduced my coughing.
The only thing that really made a HUGE difference was my discovery of Colloidal Silver last year, that quickly got rid of each chest cold and pneumonia via ultrasonc nebulizer. CS even helps my chronic asthma.
This winter I will try preventing my chronic winter chest colds with 1 ounce of Ionic Silver daily.
My new Silvertron Elite makes a gallon at a time, so I always have plenty of ionic silver on hand. In fact, I'm dropping off 30 bottles of free 20ppm ionic silver to the church this morning.