At least 37 suspects have been detained and 13 wholesalers put under investigation over a vaccine scandal that has raised deep concern about safety, Chinese media said Wednesday.

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.

From 2010, the pair illegally sold 25 different kinds of expired or improperly stored vaccines worth more than 570 million yuan ($88 million), it added.

They included shots for polio, rabies, hepatitis B and flu for both children and adults, Caijing magazine said, citing drug safety officials.

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.

"We endured melamine, we endured poisoned drug capsules, we endured milk made of leather shoes. It's as if you've killed off all the adults and so now start on the children!" cried one user on China's Twitter-like Weibo.

"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".

- 'This scares me' -

The World Health Organisation said that the expired and improperly stored vaccines were primarily a risk due to people failing to be protected from the intended diseases, rather than because of any harm they might inflict directly.

"Vaccines need to be stored and managed properly or they can lose potency and become less effective," it said in a statement.

"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," it added.

But Weibo commenters expressed deep-seated fears.

"My child is supposed to go get vaccinated, but this scares me to the point where I don't dare take him, what should I do?" asked one.

It is illegal in China for pharmaceutical companies and hospitals to do business with unlicensed wholesalers. Three pharmaceutical companies are being investigated with one ordered to halt operations, Xinhua said.

On Wednesday, a woman from the eastern province of Anhui was sentenced to three years and fined 10,000 yuan for selling fake rabies vaccines that had resulted in the death of one patient, the online Anhui News portal said, without specifying the source of her vaccines.