Dec 29 (Reuters) - Canada"s main stock index rose in thin trading on Wednesday, extending gains for a fifth straight session, helped by easing worries over the impact of the Omicron coronavirus variant. At 9:31 a.m. ET (14:31 GMT), the Toronto Stock Exchange"s S&P/TSX composite index (.GSPTSE) was up 100.35 points, or 0.47%, at 21,330.03.