The global spread of the Omicron variant of the coronavirus has brought new cases in Australia, Denmark, France and the Netherlands, prompting nations to reconsider plans for international travel as they scramble to avert an outbreak.

News of the variant triggered alarm and a sell-off last week in financial markets, as countries clamped on new curbs for fear it could resist vaccinations and upend a nascent economic re-opening after a two-year global pandemic...