A medical worker gives a polio vaccine to a child in Hami prefecture in Northwest China's Xinjiang Uygur autonomous region in an undated photo. [Cai Zengle / for China Daily]

The Shandong food and drug watchdog issued a public notice Saturday evening on hundreds of clues and identities linking 300 suspects to illegal deals of vaccine sales in 24 provinces, municipalities and autonomous regions.

Police have busted 21 cases in Inner Mongolia, Henan, Hebei and Shandong and arrested six suspects for the sales of problematic vaccines.

Among those arrested are a mother surnamed Pang and her daughter, who were charged for illegal vaccine sales worth more than 570 million yuan ($88 million).

Since 2010, the suspects purchased 25 types of vaccines from more than 100 pharmaceutical salesmen, both licensed and unlicensed and were sold to illegal agents or even local disease control and prevention centers at high prices, according to Jinan Public Security Bureau in Shandong province.

It is against the law in China to sell vaccines without proper licenses.

Though produced by qualified manufacturers, the quality of the vaccines was questionable as they were not transported in approved conditions. Such vaccines have potential side-effects and can even cause disability or death if they become expired.

The case has been transferred to the procurators.

The mother, surnamed Pang, is being arrested by the police. [Photo provided by the police]