Print Mesa Biotech, a privately held San Diego company that developed one of the first rapid COVID-19 diagnostic tests to get regulatory approval, has been acquired by life sciences giant Thermo Fisher Scientific for $450 million in cash. The deal, first announced in January, closed this week. Based in Waltham, Mass., Thermo Fisher could wind up paying up to $100 million more for Mesa if the company hits certain milestones. In March, Mesa received emergency use authorization from the U.S. Food and Drug Administration for its Accula molecular COVID-19 test system. It uses a desktop PCR analyzer device and a compact, single-use cartridge to detect the coronavirus in 30 minutes.