In the ongoing effort to understand XMRV and related retroviruses, a key concern has been assessing whether a risk exists to the nation's blood supply. In separate reports published online in the journal Transfusion, two task forces working on the issue say there's still no conclusive answer.

The task forces were set up in the wake of a study published in 2009 in the journal Science that found the retrovirus XMRV in 67% of 101 patients with chronic fatigue syndrome. XMRV was also found in around 4% of the healthy people in...