About one-in-three Covid-19 cases in the United States are now caused by the BA.2, or ‘stealth’ sub-variant of the Omicron strain of the coronavirus. This sub-variant of SARS-Cov-2 (Omicron), which is more transmissible than the already infectious omicron strain, is now found in several countries, according to the World Health Organisation. These nations, China, Hong Kong and in Europe, are also facing significant case spikes, WHO said on Thursday.  Dubbed as ‘stealth omicron’ because it presents with fewer symptoms, the sub-variant is believed to cause a more severe disease and early data shows that antibodies from omicron infection protect against reinfection with BA.2.