Washington, Feb 21 : The highly transmissible Omicron sub-variant XBB.1.5 now accounts for over 80 per cent of new Covid-19 cases reported in the US, according to the latest estimates of the Centers for Disease Control and Prevention (CDC). XBB.1.5 is estimated to account for 80.2 per cent of Covid-19 cases in the week ending February 18, up from 73 per cent in the week prior, Xinhua news agency quoted the CDC as saying in its latest update. BQ.1.1 remains the second most prevalent strain at 12.1 per cent. The CDC first started tracking XBB.1.5 in November last year, when it accounted for less than 1 per cent of cases nationwide. Since then, the strain is spreading quickly in the US. Early study suggests XBB.1.5 has a couple of concerning mutations that suggest it is even more contagious than other strains. While XBB.1.5 is spreading easier, scientists said it does not seem to cause more severe disease. /IANS