China's top health authority said on Sunday night that it is investigating a case in which children in a northwestern province may have been vaccinated with expired products.
The National Health Commission sent an investigative team to Shangluo, Shaanxi province, on Sunday, it said in a statement.
Some parents whose children received an MMR vaccine at a hospital on Mar 1, checked an app that provides information on vaccines and found that the expiration date was July 14 last year, an online post said on Friday.
MMR prevents measles, mumps and rubella, three common infectious diseases.
Another parent whose child received a DTaP vaccine in April found the expiration date was March 6, the post said.
The vaccine prevents diphtheria, tetanus and whooping cough.
The vaccine was produced in 2016 by Changchun Changsheng Bio-tech Co, a pharmaceutical manufacturer in Jilin province that is under investigation for faking production records of rabies vaccines, a serious violation discovered by the State Drug Administration last month. It also produced a substandard DTaP vaccine that may not provide effective immunization.
Some other parents said they could find no information on vaccines that their children were given for preventing epidemic meningitis or encephalitis, according to the post.
The government of Shangluo responded in a statement on Sunday morning that the city has started an investigation, and all vaccines specified in the post were within their dates of expiration.
The city government said the confusion was caused by mistakes in recording vaccine information on receivers' booklets.
The city government is investigating who may be responsible and will hold them accountable, it said.
Experts from the Shaanxi provincial health authority and center for disease control and prevention are verifying the initial conclusions of the city, it added.
Shangluo's center for disease control and prevention did not buy any substandard DTaP vaccines produced by Changchun Changsheng Bio-tech Co, the city's drug authority said last week.
An official with the city's publicity department, who asked not to be named, told China Daily that the investigation team dispatched by the National Health Commission arrived in the city on Monday, but declined to give further details.
Tao Lina, a Shanghai-based public health expert, said experience has shown that vaccines less than six months past their expiration dates can still be effective and safe.
"Vaccines are not as fragile as many imagine," he said.
It is not clear what, if any, damage long-expired vaccines can cause children, he said.