As the pandemic exploded, a researcher saw the danger. China’s leaders kept silent.

The morning of Dec. 26, 2019, began as usual at Vision Medicals in Guangzhou in southern China. This commercial laboratory, a private start-up barely a year and a half old, was also known by its Chinese name, Weiyuan Gene Technology. It specialized in next-generation sequencing, called mNGS, and offered applications that can identify most infectious agents — viruses, bacteria and others — in a single test.

A researcher browsed through the latest test results, as she did every day, before turning to her other work. She was proud of the...