/cloudfront-us-east-2.images.arcpublishing.com/reuters/IF343VLUNJNZTKLRPJCPXSNGP4.jpg "U.S. tightens COVID-19 travel rules as countries race to quell Omicron threat")
Air travellers to the United States will face tougher COVID-19 testing rules as several countries moved to seal-off their borders amid growing uncertainty around the virulence of the Omicron variant and its ability to dodge existing vaccines.
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