More than 650,000 doses of a vaccine routinely given to newborns in China have been recalled after testing showed they were ineffective.
The problem with the vaccine, which immunizes against diphtheria, pertussis (whooping cough) and tetanus, was identified after batches were distributed in Shandong and Hebei provinces and in Chongqing.
Some doses were administered before the recall, "which might diminish the immunization effect but will not threaten people's health", the China Food and Drug Administration said in a statement on Friday.
The recall relates to batches from two companies: Changsheng Biotechnology in Jilin province, which sent 252,600 doses to Shandong; and the Wuhan Institute of Biological Products in Hubei province, which sent 190,520 doses to Chongqing and 210,000 doses to Hebei.
Under the Drug Administration Law, vaccines must be checked for safety and efficacy before they can enter the market.
The National Institutes for Food and Drug Control, which carries out premarket checks, tested the DPT vaccines according to normal international practice, and reported no issues, the FDA said.
However, the ineffective vaccines were not identified.
A later routine check found the vaccines' "titer" - a measure of the concentration of antibodies - of both companies' products to be substandard.
Investigative teams have been sent to check compliance at the companies' production bases and will spend six to eight weeks testing the quality of all vaccines.
The National Health and Family Planning Commission and provincial health departments have also sent experts to assess stocks of the vaccine to ensure that routine immunization programs continue.
After the DPT vaccine was incorporated into China's immunization programme in the 1970s, the rates of diphtheria, pertussis and tetanus have fallen steadily, according to the FDA.
"There has been no diphtheria cases reported since 2007, and the annual incidence of pertussis has dropped to less than 0.5 per 100,000 people now, compared with 100 to 200 per 100,000 in the 1970s," the statement said.