Washington, Feb 25 : The highly transmissible Omicron subvariant XBB.1.5 now accounts for 85 per cent of all Covid-19 cases reported in the US this week, according to the latest estimates of the Centers for Disease Control and Prevention (CDC). The prevalence of XBB.1.5 kept increasing since late last year -- up from 79.2 per cent last week and 71.9 per cent two weeks prior, Xinhua news agency reported citing the estimates as saying. XBB.1.5 is growing in proportion in all regions nationwide, according to the CDC. BQ.1.1 remains the second most prevalent strain at 9.4 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, possibly the most transmissible one so far. While XBB.1.5 is spreading easier, scientists said it does not seem to cause more severe disease. The US still remains the worst-ht country in the world with the highest number of Covid-19 cases and deaths. As of Saturday morning, the country's overall number of cases and fatalities stood at 105,169,945 and 1,144,441, respectively. /IANS