Delta's Rapid Spread In US Clouds Summer With Fear of Covid Revival Delta's Rapid Spread In US Clouds Summer With Fear of Covid Revival While U.S. health officials say delta is on its way to becoming the country's dominant strain, an analysis by genomic testing firm Helix suggests it's already there, accounting for about 40% of new infections. Already, the variant has disrupted some nations' plans for a return to normal life. The fast-spreading delta variant is clouding Americans' hopes for a carefree summer -- and casting a shadow of doubt over plans to get back to business as usual in the fall.