A South African study offered pre-Christmas good tidings about the severity of Omicron on Wednesday as the fast-spreading coronavirus variant forces countries across the world to impose new curbs.

Governments urged citizens to vaccinate as Omicron becomes the dominant strain, upending reopening plans that many had hoped would herald the end of the pandemic, and unnerving financial markets.

Germany, Scotland, Ireland, the Netherlands and South Korea have reimposed partial or full lockdowns or other social distancing...