India, Sept. 4 -- A new Covid-19 offshoot, known as the Stratus variant, is driving up cases across the United States as infections tick upward heading into late summer. The variant, XFG, is being closely watched by health experts for its rapid spread, though officials say there's no evidence yet that it causes more severe illness.

The Centers for Disease Control and Prevention (CDC) detailed that the national test positivity rate reached 9.9 per cent in the past week, up 1.4 per cent from the week before. The sharpest increases have been recorded in the South, where test positivity rates in some states have spiked to 15 per cent.

"Currently, the most prevalent lineage in the U.S. is XFG, based on CDC's recent surveillance estimates," a...