New bird flu strain described as among ‘most lethal’ viruses

A new strain of bird flu that emerged in China during the past month is one of the ‘‘most lethal’’ flu viruses so far, worrying health officials because it can jump more easily from birds to humans than the one that started killing people a decade ago, officials of the World Health Organization said on Wednesday.

Scientists are watching the virus closely to see if it could cause a global pandemic but say there is little evidence that it can spread easily from human to human.

The W.H.O.’s top influenza expert, Dr. Keiji Fukuda, said at a briefing in Beijing that people seem to catch the H7N9 virus from birds more easily than the H5N1 strain, which began ravaging poultry across Asia in 2003. The H5N1 strain has since killed 360 people worldwide, mostly after contact with infected poultry.

Health experts are concerned about H7N9’s ability to jump to humans, and about the strain’s capacity to infect birds without causing noticeable symptoms, which makes it difficult to monitor its spread.

‘‘This is definitely one of the most lethal influenza viruses we have seen so far,’’ Dr. Fukuda said. But he added that experts were still trying to understand the virus and that there might be a large number of mild infections that were going undetected.

The H7N9 bird flu virus has infected more than 100 people in China, seriously sickening most of them and killing more than 20, mostly near the eastern coast around Shanghai. In comparison, the earlier bird flu strain, H5N1, is known to kill as many as 60 of every 100 people it infects.

The briefing on Wednesday came at the end of a weeklong joint investigation by the W.H.O. and the Chinese authorities in Beijing and Shanghai.

Experts said that they still were not sure how people were getting infected but that evidence pointed to infections at live poultry markets, particularly through ducks and chickens. They said it was encouraging that reported infections appeared to slow after the closing of live poultry markets in affected areas.

Taiwan on Wednesday confirmed its first case of the new virus, a 53-year-old man who became sick after returning from a visit to the eastern Chinese province of Jiangsu.