By Raghav Mahobe and Julie Steenhuysen (Reuters) -Over 40% of COVID-19 cases in the United States are now caused by the highly contagious Omicron XBB.1.5, data from the U.S. Centers for Disease Control and Prevention showed on Friday, with the subvariant doubling from the previous week. Although many public health experts are expressing concern about the rising COVID cases in China, infectious disease experts have been increasingly worried about the XBB.1.5 variant. “Ironically, probably the worst variant that the world is facing right now is actually XBB,” said Dr. Michael Osterholm, an infec…