Chinese health authorities detected mutations of the H1N1 virus on the Chinese mainland in June and July, but so far, the mutations are not creating any danger in terms of the number and severity of cases, health experts said.
Health officials did not make an announcement to the public earlier because the mutations carried too little public health significance.
China is among seven countries, including Brazil, Japan and Mexico, that have reported isolated cases involving this mutation, according to the World Health Organization (WHO).
Experts say the mutated virus is not circulating widely and has not been resistant to antiviral treatments like Tamiflu.
"The mutated viruses in the nation do not seem to be more virulent or infectious than the regular H1N1 one, and have caused no deaths here," said Feng Zijian, director of the emergency response department of the Chinese Centers for Disease Prevention and Control.
He declined to reveal more.
Vivian Tan, press officer of the WHO Beijing Office, said the organization had been informed by the Chinese government of the mutations earlier and that there were three such cases.
WHO's public health recommendations remain unchanged for now, as no evidence so far suggests these mutations are leading to an unusual increase in the number of H1N1 flu infections or a greater number of severe or fatal cases, the press officer said.
She urged Chinese authorities to maintain a high level of surveillance of flu-like illnesses, particularly any unusual changes in the severity or fatality of cases, to further expand the vaccination program, and to strengthen public health education among the people.
Meanwhile, Hong Kong's Department of Health announced on Monday that it had found a mutation in an H1N1 flu virus sample.
Department officials said that they had carried out an examination of the the genetic sequence of the H1N1 flu viruses in their monitoring systems.
Out of the 123 sequences studied, one sample showed a mutation.
The virus was taken from a 1-year-old boy who developed flu-like symptoms July 22.
The patient was admitted to Prince of Wales Hospital July 25 and discharged three days later. He has fully recovered.
Mutations are frequently encountered in influenza viruses, according to WHO.