It comes after police in Shandong announced last month they had arrested a mother and her daughter accused of buying and selling vaccines illegally.
The estimated $88m (Â£61m) worth of vaccines were not properly refrigerated or transported.
The illegal vaccine ring, said to have been in operation since 2011, has sparked widespread anger in China.
The scandal has led to a crackdown, with checks ordered on vaccine makers, wholesalers and buyers.
The vaccines were bought from a variety of sources, both licensed and unlicensed, then sold on to both illegal agents and legal local disease control and prevention centres at inflated prices, reported the Xinhua news agency.
The ring is alleged to have involved hundreds of people across 20 provinces, it said.
Though authorities had known about it since April last year, they only made the news public late on Friday when they issued a call demanding that suppliers come forward to help them trace potential victims.
The delay has angered many people in China who questioned why the authorities had not alerted the public earlier.
Local authorities have now been given a deadline of Friday to identify who bought the medicines.
Three pharmaceutical companies are being investigated, Xinhua reports, citing people handling the case. One, Shandong Zhaoxin Bio-tech Co, has been ordered to suspend operations, it added.
The China office of the World Health Organization (WHO) said vaccines needed to be handled properly or they can become less effective.
But it stressed that improperly kept vaccines did not in themselves present much danger.
"It is important to note, however, that improperly stored or expired vaccine seldom if ever causes a toxic reaction. Therefore there is likely to be minimal safety risk in this particular situation,'' the WHO said.

Plans to employ a marksman to shoot the pied wagtail at the Great Yarmouth store were put on hold after TV star Chris Packham tweeted his opposition.
The British Trust for Ornithology said it had been caught, with Mr Packham tweeting a photo of the "renegade".
Tesco said shooting the bird was only considered as a last resort.
Natural England granted the firm a licence to shoot the wagtail earlier this month on the grounds of public health after it was spotted in the shop's aisles.
But Mr Packham, who presents BBC Springwatch, used his Twitter account to urge Tesco to get in touch with the BTO instead, to attempt to catch the bird safely.
Tesco said many attempts to catch it had failed, but tweeted it was "happy to liaise with BTO and explore other options".
The BTO confirmed it had been caught earlier after putting up a giant net in the store.
Pied wagtails are typically 18cm (7in) long and weigh 21g (three-quarters of an ounce). There are 470,000 breeding pairs in the UK.
They are protected by the Wildlife and Countryside Act, with anybody who kills one without a licence facing a Â£5,000 fine.
Thirty-seven people have been arrested in eastern China over a huge vaccine scandal, state media report.
[NEXT_CONCEPT]
A small bird that earned a last minute reprieve after supermarket giant Tesco retracted a bid to shoot it has been caught and released.