Four employees at a public clinic in Chongqing, Southwest China, were arrested on Sunday for making illegal profits from suspect vaccines.

Last Friday, a post about a clinic nurse in Nan'an District suspected of replacing expensive, imported vaccines with fake ones went viral online. Hundreds of angry parents protested over the weekend demanding a thorough investigation into vaccine management at the clinic, which has been providing services to the community for years.

The clinic is under the management of Chongqing No 6 People's Hospital.

Concerns were first raised when a mother noticed a nurse at the clinic using different colored syringes to administer a 700 yuan ($108) imported pentavalent vaccine, which is intended to protect children from five potentially deadly diseases including tetanus, hepatitis B and diphtheria.

According to the online post, the nurse would sometimes use blue syringes that came with the vaccine and at other times use white disposable syringes when giving the shots.

Many parents commented that their children had suffered an adverse reaction to the vaccine and suspected foul play.

A health expert with the team investigating the incident told the media on Sunday evening that it was acceptable for the clinic nurse to use disposable syringes if the original ones had become contaminated.

However, the team also found that four employees at the clinic, including the nurse, had illegally profited from falsifying vaccine purchase and registration records.

Last month, a revision of the previous regulation on the management of vaccines was approved by the State Council, China's Cabinet, after a scandal involving 570 million yuan worth of Category 2 vaccines that had been stored improperly and sold across China.

This revision intensifies management rules for vaccines, including their transportation and storage, and increases punishments for violators.

It also stipulates that chief government officials should resign if serious vaccine violations occur within their jurisdictions.