Countries around the world reported their first cases of the Omicron variant over the weekend and some imposed new travel restrictions, racing to protect themselves against the potentially more contagious strain even as scientists cautioned they don’t yet know how severe it will prove to be.

Canada, Australia, the Netherlands and Austria on Sunday joined a group of countries that have detected a strain first documented in South Africa that authorities said could pose a greater risk of people falling ill with Covid-19 a second time and could be more transmissible than other variants.