JOHANNESBURG — Dozens of countries restricted travel to and from South Africa and neighbouring nations on 26 November, hoping to contain a fast-spreading new variant of the coronavirus that scientists say may be more contagious and could render the current crop of Covid-19 vaccines less effective.

Experts from the World Health Organisation met on 26 November to decide whether to declare the new strain — currently known as B.1.1.529 — a “variant of concern.” The WHO uses this label for virus strains that have been proven to be more contagious, lead to more serious illness or decrease the effectiveness of public-health measures, tests, treatments or vaccines.