BEIJING (Reuters) -China’s northern port city of Tianjin reported an increase in COVID-19 infections on Thursday as it stepped up efforts to rein in an outbreak that has spread the highly transmissible Omicron variant to another city. Omicron has brought new challenges for China’s strategy to quickly stamp out outbreaks, which has taken on urgency ahead of the Winter Olympics set to start from Feb. 4, while the busy Lunar New Year travel season begins this month. Volkswagen Group’s China unit said it had shut a vehicle plant run jointly with FAW Group in Tianjin, as well as a component factory…