HONG KONG — The police in China say they have uncovered a criminal ring suspected of selling improperly stored vaccines in dozens of provinces.

After the arrest of a former doctor and her daughter in Shandong Province, in eastern China, the police said they were searching for 300 suspects across the country.

More than 20 people have been detained since last year or are on bail in connection with the case, the state news agency Xinhua reported on Saturday.

The former doctor in Shandong was convicted of illegally selling vaccines in 2009 and was given a suspended three-year prison sentence, according to The Paper, an online news organization that first reported on the case. But rather than halt her activities after her conviction, the suspect, who has been identified by only her surname, Pang, apparently enlisted her daughter to expand the vaccine sales, officials said.