Global Covid-19 cases surpassed 400 million on Wednesday, according to a Reuters tally, as the highly contagious Omicron variant dominates the outbreak, pushing health systems in several countries to the brink of capacity.

The Omicron variant, which is dominating the surge around the world, accounts for almost all new cases reported daily.

While cases have begun to level off in many countries, more than 2 million cases are still being reported on average each day, according to...