SHANGHAI • The Chinese authorities have launched a crackdown on the black-market sale of vaccines following reports that illegal vaccines worth 570 million yuan (S$120 million) have been sold in more than 20 provinces and cities across China.

The drug regulator in Shandong, the province at the heart of the scandal, said yesterday it would work with police forces and the Health Ministry to inspect vaccine stocks to ascertain where the vaccines had ended up.

The case, according to Chinese media, is the largest of its kind on the mainland in terms of the amount of money involved.

At the centre of the scandal are a mother and daughter from Heze city of Shandong province. Both women have allegedly been involved in the illegal sale of 25 kinds of vaccines - for adults and children - since 2010.

The mother, identified by her surname Pang, formerly worked as a pharmacist at a hospital in Heze, Shandong. She also ran a clinic selling vaccines. She was reportedly well known for her close ties with pharmaceutical firms, reported Hong Kong's South China Morning Post.

In 2009, the mother was given a three-year jail sentence, suspended for five years, for illegally selling vaccines, reported local media. Yet she allegedly continued the illegal business and was joined by her daughter - a medical school graduate - in 2014 .

The case underlines the challenge the world's second-largest drug market faces to regulate its fragmented supply chain even as Beijing looks to support home- grown firms, reported Reuters.

Shandong police said the pair had illegally bought vaccines from traders and sold them on to hundreds of re-sellers around the country.

The vaccines, which police said were made by licensed producers, were not kept and transported in the required cold-chain conditions, reported local media.

Health experts warned that patients taking these vaccines could suffer severe side effects or even death.

Vaccines must be kept at temperatures between 2 deg C and 8 deg C, according to Chinese media.

The vaccines include those for flu, chicken pox, hepatitis A, meningitis and rabies.

The mother and daughter were detained last April, but the case was not widely publicised until now.

Last Saturday, Shandong's food and drug department released a list of 107 suspects who allegedly supplied the pair with the vaccines. The department also made public another 193 suspects who allegedly bought vaccines from the mother and daughter.

Provincial drug authorities are tracking down the list of suspects, reported the Post.

China's national food and drug regulator also called on authorities at the provicial level to investigate the scandal.

In a statement posted online on Sunday, the regulator urged all suspects in the case to turn themselves in and reveal the whereabouts of the vaccines by Friday, reported China Daily.

Beijing municipal health authorities said on Saturday that all vaccines sold in the capital's regulated medical centres were "safe" as they had all been properly registered and transported.