The Omicron variant, first sequenced in South Africa, has been identified in at least 12 other countries.

Most have so far reported imported cases spotted in travellers from the African nation and other places. Still, the spread shows the difficulties of curtailing new strains of Covid-19.

Israel, for instance, said a confirmed case who arrived from Malawi rode on a bus from Tel Aviv. Italy's first case travelled around the country for days before testing positive.

Researchers worldwide are racing to understand the full impact of the new strain and governments have banned travellers from South Africa and nearby countries on concerns Omicron could evade the protection of vaccines or fuel new Covid surges.