Masked commuters walk through a walkway in between two subway stations as they head to work during the morning rush hour in Beijing, on Dec. 20.Andy Wong/The Associated Press Could the COVID-19 surge in China unleash a new coronavirus mutant on the world? Scientists don’t know but worry that might happen. It could be similar to omicron variants circulating there now. It could be a combination of strains. Or something entirely different, they say. “Chinahas a population that is very large and there’s limited immunity. And that seems to be the setting in which we may see an explosion of...