LONDON—The newly-discovered Omicron variant shows policymakers and financial markets cannot lower their guard on COVID-19 and will have to calibrate their policies carefully, the Bank for International Settlements said on Monday. Dubbed the central bank to the world’s central banks due to its regular gatherings of decision makers, the Swiss-based BIS said Omicron had already caused falls in major stock markets and ramped up uncertainty. “The emergence of Omicron indicates that we should not lower our guard,” Claudio Borio, head of the BIS’ Monetary & Economic Department told reporters. “This was the latest reminder that we have to be watchful.” As uncertainty rises over the potential human and economic costs of the new variant, global financial markets are also waiting to see whether surging inflation drives major central banks like the U.S. Federal Reserve, Bank of England, and European Central Bank to raise interest rates. Financial conditions have already been …