JOHANNESBURG, Dec 8 (Reuters) - The South African rand extended this week"s gains on Wednesday, as signs that the Omicron coronavirus variant identified locally and in Hong Kong is causing mainly mild infections supported risk appetite. At 1445 GMT, the rand traded at 15.7950 against the dollar, 0.35% stronger than its previous close. Financial markets have been highly sensitive to news about Omicron, which South Africa alerted the world to last month, triggering global alarm and the imposition of travel restrictions. But anecdotal accounts from South African doctors and researchers suggest that it may be causing less severe clinical symptoms than other coronavirus variants, though they caution that more research is needed before definitive conclusions can be drawn. read more Markets also cheered news that BioNTech and Pfizer (PFE.N) said that a three-shot course of their COVID-19 vaccine was shown to generate a neutralising effect against the new Omicron variant in a laboratory test. read more Also supporting sentiment, a proposal to change the constitution to explicitly allow expropriation of land with no compensation failed to win sufficient votes in parliament on Tuesday. Opposition parties and some investors have seen the proposal as threatening property rights. read more Weighed down by local data, the main indexes on the Johannesburg Stock Exchange moved away from record highs reached in the previous session, with the benchmark all-share index (.JALSH) finishing 0.72% lower at 72,403 points, while the blue-chip index of top 40 companies (.JTOPI) ended 0.78% lower at 65,992 points. Retail sales were up 1.8% year on year in October, slightly lower than September"s 2.1% reading, Statistics South Africa said on Wednesday. "Despite the modest annual performance, retail trade sales contracted on a month-on-month seasonally adjusted basis, following September"s 5.1% rise. Indeed, consumers remain financially constrained, and many have not recovered from the effects of the pandemic," Investec Bank Economist Lara Hodes said. Weighing on stocks as well was data on South African business confidence, which fell in November compared to the previous month as both exports and imports dipped. The retail sector has had a mixed year, as it was one of the major casualties from July riots that contributed to an economic contraction in the third quarter.