On Sunday, Mo Li, 27, was browsing Weibo when she saw the news: 250,000 defective vaccines had been administered to children in the province where she and her husband are raising their only child.

“It was like an explosion in my head,” says Mo, who lives in the eastern port city of Weihai, in Shandong province. “My only thought was, ‘please not my son, please not my son’,” she said, describing her thoughts while searching for her 17-month-old’s health records.

The serial number on his vaccination record matched that of the batch in question, made by one of China’s largest vaccine makers, Changsheng Biotechnology. Mo looked at her son, Congcong, asleep, and felt awful and then angry.

“I thought about all the people involved, from the vaccine company to the regulators. They cannot be called human. They are devils in hell,” she said.

Like Mo, thousands of parents across China have been scrambling for answers this week after revelations their children may have received faulty immunisations under a state-sponsored vaccine program, using products from Changsheng. The incident has sparked one of the country’s largest public outcries in years, one that officials and censors have struggled to contain.

Changsheng, a private company based in Jilin province, is under multiple investigations for fabricating inspection records for a rabies vaccine as well as selling at least 250,000 substandard DPT vaccines – for diphtheria, whooping cough, and tetanus – to health clinics in Shandong. Fifteen people, including the chairwoman of the company, have been arrested on “suspicion of criminal offences” while China’s top graft investigation agency has said it will investigate.

Chinese president Xi Jinping, who rarely comments on such public cases, promised the government would “investigate to the end”. “It is necessary to promptly release the progress of the investigation and effectively respond to the concerns of the people,” he was quoted as saying, according to Chinese state media on Monday.

A recurring issue

There have been no reports of injuries from the defective vaccines and officials promised children would receive new vaccines. But public trust has hit a new low: people say they have heard this all before.

As China has expanded its immunisation program over the past decade, substandard vaccines have been a recurring issue. In 2016, $90m in vaccines were found to be stored improperly in Shandong province. The year before, hundreds of children in Henan province reportedly fell ill after being given out-of-date vaccines. In 2010, a newspaper in Shanxi province reported unrefrigerated vaccines had killed four children.

A hospital worker walks past a poster with information on rabies vaccine in Beijing, China. Photograph: Ng Han Guan/AP

Many are recalling another public health crisis, in 2008, when tainted milk powder killed six infants and left 300,000 others ill. Officials initially tried to cover up the incident. In the case of Changsheng, inspectors discovered the substandard vaccines last year, but they were not pulled until this month.

Patrick Poon, China researcher for Amnesty International, says: “The vaccine scandal once again shows the Chinese government’s failure in law enforcement and monitoring the healthcare industry, even after years of reports about other cases.”

Angry parents, like Mo, have been emboldened. On Monday, she and a group of mothers confronted doctors at the local health clinic. They wanted to know whether the faulty vaccines were dangerous, if they would need new ones, and if so, how to ensure those wouldn’t hurt them.

They were told: wait for official instructions. The group stayed at the clinic for two hours before leaving. “We don’t accept any apology. We don’t want compensation. We just want our babies to be healthy!” Mo posted online.

The fault is first and foremost on the regulators. Qi Jing, father of vaccinated child

Censors struggle against ‘destructive elements’

On social media, censors failed to stem the deluge of criticism. According to Weiboscope, a project by the University of Hong Kong monitoring censorship, “vaccine” was one of the most restricted words on Weibo on Sunday and Monday. By Tuesday, related hashtags had been viewed more than 600m times.

A poll on Weibo asked whether people would use Chinese-made vaccines in the future: 90% of more than 700 respondents chose the answer, “I wouldn’t dare use them again.”

Such open public debate will likely be allowed to go on for only so long. State-run Global Times said in an editorial on Tuesday, “If there is no management of public opinion, it can fester and lead the country to chaos ... We must have the ability to prevent destructive elements from spreading online.”

Qi Jing, a single father in Chongqing, in south-western China, says his three-year-old daughter received vaccines from Changsheng and another company implicated this year for faulty vaccines, Wuhan Institute of Biological Products. Following the news of Changsheng’s counterfeits, he organised a group of about 60 parents to demand information. He was detained this week and questioned by police.

“Right now what the government does is conceal, and control public opinion. Making any sound is difficult,” Qi says. He adds police advised his group to focus on the companies rather than the government.

“The fault is first and foremost on the regulators. If they were looking after public health as they should, companies couldn’t produce counterfeits and would have no choice but to be responsible,” he says.

For now, parents have little choice but to continue immunising their children, as required by Chinese law. Mo is not sure what to do, but she and her husband have made one decision.

“After this, I’m definitely not having another child. I cannot protect two when I’m not even able to protect one.”

Additional reporting by Wang Xueying