Most of the problematic vaccines involved in a recent illegal sale case have been used, and flaws in supervision of vaccine distribution are to blame, officials said at a news conference on Thursday.

Shandong police announced recently that they had arrested a mother and daughter alleged to have illegally sold improperly stored or expired vaccines worth more than 570 million yuan ($87.5 million) across 20 provinces since 2011.

About 20,000 unused doses of the vaccines have been found so far, and investigations are underway to locate other vaccines, said Hua Jingfeng, deputy director of the Public Security Management Department at the Ministry of Public Security.