Health workers obtain samples from pigeons at a village in Cuigezhuang township, Chaoyang district, Beijing, on Monday. A 4-year-old boy from the village was found to have the H7N9 virus. Hou Yu / China News Service

A 4-year-old boy in Beijing confirmed as an H7N9 flu virus carrier who has not yet shown symptoms may be an indication that some people are immune to the disease, a health official has said.

The Beijing Municipal Health Bureau said in a statement on Monday that the boy, surnamed Zhu, tested positive for the H7N9 flu virus at the Beijing Center for Disease Control and Prevention. The agency suspected his case might have been caused by chicken that was resold by the family of a 7-year-old girl who became Beijing's first H7N9 case on Saturday.

Fight against the H7N9 bird flu

The boy, who is being monitored at Beijing Ditan Hospital, has not shown any symptoms of the virus, a possible indicator of immunity, said Zhong Dongbo, deputy chief of the municipal health bureau.

"If we can find a large number of carriers of the virus who show no symptoms, that means an immunity barrier could exist among human beings and the virus could disappear eventually," Zhong told a news conference.

Because many cases of infection in East China have been severe, the diagnosis of the boy who has shown no symptoms indicates that the H7N9 virus can also lead to mild or even no symptoms. Confirmation of the boy as a virus carrier also further corroborated the source of the infection as poultry, he said.

Local health authorities traced the boy's case to the village he lives in. The father of the first person infected in Beijing, the 7-year-old girl, sold poultry to villagers. Authorities took throat swabs from 24 people with a high risk of getting the virus, and the city's CDC found the virus in the boy's specimen.

A family that lives across the street from the boy's house bought chicken from the father of the 7-year-old girl, but health authorities failed to find the cause of the 4-year-old's infection because the chicken had already been eaten, he added.

Despite the health bureau's report, Jiang Rongmeng, chief physician at the Infection Center of Ditan Hospital, believes it is premature to call the boy a virus carrier.

"When people infected have a strong immunity, they will naturally recover. The other possibility is an apparent infection like hepatitis B," he said. "We need to further observe this flu virus to see how it will develop inside humans."

The boy's case was discovered as disease control and prevention authorities in Chaoyang district stepped up efforts to monitor a group of high risk from the H7N9 virus, which is mainly poultry industry workers.

As in the girl's case, the parents of the boy trade poultry in Naidong village, Cuigezhuang township in Chaoyang district.

The boy's parents and his elder sister are also under medical observation at hospitals, and the other people who were given tests were under watch in the village.