Harrington, one of the principle researchers in the SCCHN trials, published some work about a year ago showing how Reo attaches to blood cells in a manner that allows it to hide from the immune system.
Shortly after the article was published, the several NCIC trials were announced.
Its not about immunity, its about getting enough Reo in the system. The dosing of Reo in these trials is far more than what one would experience in the normal course of natural Reo infection.
The current thinking on use of chemo is based on the supposition that chemo makes the tumors more porous and permeable to Reo infiltration.