GENEVA — The Omicron subvariant, XBB.1.5, is causing concern among scientists after its rapid spread in the US in December. Here is what we know so far: The World Health Organisation (WHO)'s senior epidemiologist Maria Van Kerkhove said XBB.1.5 is the most transmissible Omicron sub-variant that has been detected so far. It spreads rapidly because of the mutations it contains,...