FRANKFURT—European companies playing key supporting roles in COVID-19 vaccine manufacturing are working to move production and supply chains closer to their customers to guard against trade restrictions that have interrupted supplies during the pandemic. Germany’s Merck KGaA, whose Life Science unit is one of the world’s largest makers of bioreactor gear and supplies, told Reuters it is pushing to spread its production network geographically so that fewer shipments have to cross customs borders. U.S. regulations in particular, which give priority to companies fulfilling U.S. government contracts, have posed a challenge for Merck as its seeks to meet soaring demand for supplies such as sterile fermentation bags and filters. But the United States is not the only country engaging in what some call vaccine nationalism. India barred vaccine exports in mid-April to focus on its domestic immunization drive. In the wake of production shortfalls at AstraZeneca earlier this year, the European …