Dec 22 (Reuters) - Canada"s main stock index opened lower on Wednesday, retreating from the rally a day earlier, as investors remained cautious amid renewed global restrictions to curb the spread of Omicron variant cases, with healthcare shares leading losses. At 9:31 a.m. ET (14:31 GMT), the Toronto Stock Exchange"s S&P/TSX composite index (.GSPTSE) was down 44.47 points, or 0.21%, at 20,880.4.