By Kate Kelland
LONDON (Reuters) - A new and deadly strain of bird flu that emerged in China in February but seems to have petered out in recent months could reappear later this year when the warm season comes to an end - and could spread internationally, scientists said on Monday.
A study by researchers in China and Hong Kong found only one human case of the H7N9 bird flu strain has been identified since early May.
In the preceding months, the virus, which was unknown in humans until February, has infected more than 130 people in China and Taiwan, killing 37 of them, according to the World Health Organization (WHO).
"The warm season has now begun in China, and only one new laboratory-confirmed case of H7N9 in human beings has been identified since May 8, 2013," the researchers wrote in a study published in The Lancet medical journal.
But they added: "If H7N9 follows a similar pattern to H5N1, the epidemic could reappear in the autumn."
H5N1 is another deadly strain of bird flu which emerged in 2003 and has since spread around the world. Latest WHO data on H5N1 show it has killed 375 of the 630 people confirmed as infected in the past 10 years. Many H5N1 cases have been in Egypt, Indonesia and Vietnam.
The researchers, from the Chinese Center for Disease Control and Prevention (CDC) in Beijing and the University of Hong Kong, said the potential lull in H7N9 could offer health officials the chance to properly discuss and plan ahead for the possibility of the flu's return and wider spread.
This should include plans to build healthcare capacity in the region "in view of the possibility that H7N9 could spread beyond China's borders," they said.
Experts from the United Nations agency said last month the bird flu outbreak in China had cost the economy some $6.5 billion.
In a second study published in the same journal, the researchers also found that while H7N9 flu has a lower risk of death than its much-feared cousin H5N1, it has a higher fatality risk than the 2009 H1N1 flu which swept the world in 2009 and 2010 in a pandemic.
After analyzing data on hospital admissions, the team found that H5N1 bird flu had a fatality risk of around 60 percent for patients admitted to hospital - almost double that of the new H7N9 strain which has a death rate of about a third of those hospitalized with the infection.
Pandemic H1N1, often referred to as "swine flu", killed 21 percent of those it infected who were taken into hospital, the researchers said.
The team urged health officials and doctors not to be lulled into a false sense of security by the sharp drop off in H7N9 cases in recent weeks.
"Continued vigilance and sustained intensive control efforts against the virus are need to minimize risk of human infection, which is greater than previously recognized," they said.
(Reporting by Kate Kelland; Editing by Robin Pomeroy)