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Experts from the UN's health agency are examining whether the H7N9 bird flu virus is spreading among humans, after a cluster of cases among relatives, but downplayed fears of a pandemic Friday.
"What we don't know is the size of the iceberg under this tip," said the World Health Organisation's representative in China after revealing details of three families who have shown possible human-to-human transmission.
Michael O'Leary was speaking as 15 global international health experts began a weeklong mission in Beijing and Shanghai to investigate the H7N9 bird flu virus, which has killed 17 people and sickened 70 others.
"The primary focus of the investigation is to determine whether this is in fact spreading at a lower level among humans. But there is no evidence for that so far except in these very rare instances," he added.
He said one family in Shanghai had shown evidence of more than one family member being infected with the deadly strain.
In the two other clusters, one family member was infected while the other had "clinically similar and presumed H7N9".
He said investigators were trying to determine whether the family members were infected with the virus from the same source, or from each other.
Since China announced nearly three weeks ago that it had found the strain in people for the first time, almost all of the cases have occurred in Shanghai and four nearby provinces while one appeared in Beijing.
O'Leary also said investigations into the source of the virus were continuing, though the virus is believed to be crossing to humans from birds, prompting mass poultry culls in several cities.
"The evidence suggests still that poultry is a vehicle for transmission but epidemiologists haven't yet been able to establish a clear and strong link," he said.
He also said that "more than half" of those with the virus remembered coming into contact with birds.
O'Leary again reiterated that authorities had offered the WHO unrestricted access in the investigation.
In 2003 China was accused of trying to cover up the outbreak of Severe Acute Respiratory Syndrome, which went on to kill about 800 people worldwide. But it has received praise for its transparent handling of H7N9.
O'Leary's remarks came two days after a senior official from China's Centre for Disease Control and Prevention stressed that the family clusters did not necessarily indicate that one relative had spread the virus to another.
"Even though there are a few family cluster cases, it does not mean the virus has the ability to effectively transmit from human to human," said Feng Zijian, according to the transcript of a press briefing.
A seven-year-old girl who contracted the strain left hospital on Wednesday, but since then a group of men have been stationed near her home to prevent her from straying too far, the Beijing News said on Friday.
The guards have instructions to keep the family from entering crowded places, and even run errands for them.
Another H7N9 patient, a 51-year-old woman in eastern Zhejiang province, was released from hospital, the Xinhua news agency said on Friday.
With no vaccine for the virus for now, some people have turned to traditional Chinese medicine to guard against it, although experts warned that these might help to treat rather than prevent the disease, if anything.
Nearly a third of H7N9 patients so far have received such remedies, the news agency Xinhua said on Wednesday, citing the State Administration of Traditional Chinese Medicine.
A pharmacist surnamed Zhu in Shanghai, where the largest number of cases has been found, said customers had been buying a traditional Chinese herb for colds called banlangen, which they believe gives protection from the virus.