China's Health Ministry is probing a report in a domestic newspaper that faulty vaccines in northern Shanxi province were responsible for killing four children and making dozens of others sick.
China has been beset by a series of product safety scandals over the past few years. At least six children died in 2008 after drinking milk contaminated by the industrial chemical melamine.
In 2003 and 2005, three Chinese children suffered severe brain damage after being vaccinated against Japanese encephalitis. Their parents blamed a substandard vaccine, something the government denied.
In the latest case, the China Economic Times reported four children died and at least 74 others became sick in the poor inland province of Shanxi, after getting vaccinated against illnesses including encephalitis, hepatitis B and rabies.
The problems arose when the vaccines were spoiled by the summer heat, the report said. It also gave a list of the names of some of the children it said had either fallen sick or died.
The Health Ministry said it was checking the report.
"(We have) immediately begun an investigation and demanded the Shanxi health authorities report as soon as possible any new abnormal reactions to the vaccines," it said in a statement on its website (www.moh.gov.cn) late on Wednesday.
State news agency Xinhua quoted Li Shukai, deputy head of the Shanxi Health Department, as denying the accusations.
The ministry added that earlier reports about bad vaccines had prompted checks in November 2008, but that it had found no problems.
Rare complications can happen with any vaccine and it is usually hard to pin down the reason, which may relate to the quality of the vaccine or the person's own immune system.
The proper storage and refrigeration requirements for many types of vaccines sometimes present a problem when they need to be transported across vast distances.
The reporter who wrote the story, Wang Keqin, told Reuters he stood by his piece, saying it was the result of months of thorough investigation.
"What I wrote was based on evidence," Wang said.
China, often called the world's factory, is struggling to convince a skeptical domestic and global audience it has won a battle to improve safety standards after scandals involving everything from pet food and dumplings to cough syrup.
In 2007, China executed former drug and food safety chief Zheng Xiaoyu. His misdeeds led to approval of many medicines that should have been blocked or taken off the market, including six fake drugs, a court found.