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]

BEIJING, March 22 -- China's drug regulator has identified nine vaccine wholesalers suspected of filing fraudulent reports of buyers' identities.

The China Food and Drug Administration has given local authorities until Friday to find out who the real buyers of the vaccines were. It said it will make the results public later this week.

The wholesalers, based in six provinces, came to light as police attempt to trace vaccines worth more than 570 million yuan (88 million U.S. dollars) allegedly sold illegally since 2011 by a mother and daughter.

Police in Shandong Province announced last month that they had arrested the pair.

All the vaccines involved were produced by licensed manufacturers, but the authorities fear their quality may have been compromised if they were not transported and stored in appropriate conditions.