Health officials say they still do not understand how a lesser-known bird flu virus was able to kill two men and seriously sicken a woman in China, but that it is unlikely that it can spread easily among humans.
Two men in Shanghai became the first known human fatalities from the H7N9 bird flu virus after contracting it in February. A woman in Chuzhou, Anhui Province, remains in serious condition, China’s National Health and Family Planning Commission said.
It was unclear how the three patients became infected, it said.
It sought to calm fears about the virus, but provided few details about each case. It said that two sons of one of the Shanghai men also suffered from acute pneumonia, and the source of their infection is still unknown.
The commission said other people who were in close contact with the victims have not become sick, indicating that the virus is not easily transmitted between humans.
“We don’t know yet the causes of illness in the two sons, but naturally, if three people in one family acquire severe pneumonia in a short period of time, it raises a lot of concern,” WHO China representative Michael O’Leary said at a briefing in Beijing yesterday.
Other strains of the H7 family of bird flu viruses have caused mostly mild human infections in the past, said University of Hong Kong microbiologist Malik Peiris, with cases reported in the Netherlands, Canada, the US and Britain — mostly following outbreaks in poultry.
Experts say the deaths in China might indicate that the H7N9 strain has morphed to become more lethal to humans, although it is not possible to make any conclusions yet about its mortality rate because many mild cases may go undetected. A thorough tracing of the virus is critical.
“I would guess that given the severity of the human disease it is likely that these particular viruses have undergone the change to become highly pathogenic, but obviously that remains to be ascertained,” Peiris said. “The crucial question is the source of this virus, where is it.”
“At the moment, there has been not much evidence of human-to-human transmission [of H7N9] so to that extent it is similar to the H5N1 situation, but it is early days and so there’s a lot more to be understood,” Peiris said.
Scientists classify flus based on the proteins on the surface of the virus: There are 17 varieties of hemagluttinin, the H in a flu’s name, and 10 varieties of neuraminidase, the N component. Any combination of those Hs and Ns could crop up and potentially mutate into a form that is spread easily from person to person, making it dangerous enough to produce a pandemic.
Health authorities are monitoring 88 people who came into contact with the H7N9 patients and have not found any additional infections so far, China’s health agency said.
Experts say that indicates that the chance of human-to-human transmission is low.
“It is very unlikely, because the virus has to break the species barrier and this is usually quite a difficult event. There has to be a lot of significant mutation,” said David Hui (許樹昌), an infectious disease expert at the Chinese University of Hong Kong.