Hong Kong will expand stool testing for Covid-19 among the very young and the elderly in the wake of a local university study that found the coronavirus in samples taken from patients with no respiratory symptoms – in some cases, days after they tested negative.The two-part study, carried out by Chinese University, showed that collecting stool samples was a better option for infants and the elderly, who might find it difficult to undergo nasal or throat swabs, scientists said.The study covered…