China's top drug regulator said on Tuesday that a temporary equipment malfunction led to more than 400,000 ineffective vaccines, often used for infants and children, being sold in two provincial regions.

A total of 400,520 doses of DTaP vaccine made in 2016 by Wuhan Institute of Biological Products Co, in Wuhan, Hubei province, were sold in Chongqing municipality and Hebei province.

The vaccine can prevent diphtheria, tetanus and whooping cough in infants.

Based on an inspection of the production site and the company's analysis, it can be concluded that the primary cause of the substandard vaccines was a temporary malfunction of equipment that resulted in an uneven ratio of liquid in doses to be packaged, the State Drug Administration said in a statement on Tuesday.

The administration said inspectors found that the vaccines were safe but may not be effective in preventing whooping cough.

The administration dispatched investigative teams to the company following discovery of substandard products.

Meanwhile, the company recalled all unused vaccines and destroyed them under the supervision of the local drug authority in May, it said. The company received punishment such as fines, it said.

Following the incident, the SDA inspected all 30 batches of DTaP vaccine the company later produced and found they were up to standard, it said.

Health authorities in both Hebei and Chongqing announced earlier this month that they had found no signs of major health risks associated with the substandard vaccines, and said they would administer new vaccinations to any children who had received the substandard product.

An official of Wuhan's food and drug administration said most of the substandard vaccines produced by the Wuhan company had been used, Chengdu Economic Daily newspaper reported on Thursday.

What caused the substandard vaccines wasn't known despite multiple inspections by the State Drug Administration, the newspaper quoted the official as saying.

In addition to the 400,520 DTaP doses, another 252,600 doses of ineffective DTaP vaccines made by Changchun Changsheng Bio-tech Co, in Chuangchun, Jilin province were sold in Shandong province, the drug administration said in November.

Most of the vaccines had been used, according to the Shandong provincial center for disease control and prevention.

Song Lizhi, director of the Shandong CDC's immunology department, attempted suicide by injecting excessive insulin into his body and is currently hospitalized, Beijing-based China Times reported on Tuesday, citing anonymous sources.

It is not known whether the case is linked with the vaccine scandal in Shandong province.

The report about Song had not been confirmed as of Tuesday night.

Shandong's top anti-graft authority said last week that it is investigating the flow of substandard DTaP vaccines into the province, focusing on possible violations of the law, as well as possible corruption related to supervision.