The case involves the illegal and improper storage, transport and sale of tens of millions of dollars' worth of vaccines -- many of them expired, the official Xinhua news agency said.
The incident is the latest health and safety scandal to emerge in China, where 300,000 children fell ill, six of them dying, in a notorious 2008 case involving milk powder contaminated with melamine.
The two linchpin suspects in the latest scare are a mother and daughter duo from Shandong province in eastern China, Xinhua said, citing investigating officials.
They included shots for polio, rabies, hepatitis B and flu for both children and adults, Caijing magazine said, citing drug safety officials.
Also Read
At the weekend, the Shandong Province Food and Drug Administration identified 107 wholesalers who supplied the pair, and 193 retailers across the country who were their clients.
Public fury has erupted over authorities' delay in publicising the case, which only came to light this month despite the pair being arrested in April 2015, almost a year ago.
"All those who've committed such crimes and intentionally killed people should be shot! The ministry of health is nothing but a decorative ornament," she added.
Another lamented: "What we want is cheap housing, free schooling, free medical care, non-toxic food, non-toxic water and non-toxic air like there is in other countries."
Premier Li Keqiang acknowledged in a statement on Tuesday that the incident had provoked "great concern and exposed the existence of many regulatory loopholes".