Scientists have reported the first case of human-to-human transmission of the new strain of bird flu that has emerged in China.
The British Medical Journal said a 32-year-old woman was infected after caring for her father. Both later died.
Until now there had been no evidence of anyone catching the H7N9 virus other than after direct contact with birds.
But experts stressed it does not mean the virus has developed the ability to spread easily between humans.
By 30 June there had been 133 cases of H7N9 bird flu reported in eastern China and 43 deaths.
Most people had visited live poultry markets or had close contact with live poultry in the week or two before they became ill.
Yet researchers found that the 32-year-old woman had become infected in March after caring for her 60-year-old father in hospital.
Scientists have reported the first case of human-to-human transmission of the new strain of bird flu that has emerged in China
Unlike her father – who had visited a poultry market in the week before falling ill – she had no known exposure to live poultry but fell ill six days after her last contact with him.
Both died in intensive care of multiple organ failure.
Tests on the virus taken from both patients showed the strains were almost genetically identical, which supports the theory that the daughter was infected directly from her father rather than another source.
Public health officials tested 43 close contacts of the patients but all tested negative for H7N9, suggesting the ability of the virus to spread was limited.
The researchers said that while there was no evidence to suggest the virus had gained the ability to spread from person to person efficiently, this was the first case of a “probable transmission” from human to human.
“Our findings reinforce that the novel virus possesses the potential for pandemic spread,” they concluded.
Dr. James Rudge, of the London School of Hygiene and Tropical Medicine, said that limited transmission between humans is not surprising and has been seen before in other bird flu viruses, such as H5N1.
He added: “It would be a worry if we start to see longer chains of transmission between people, when one person infects someone else, who in turn infects more people, and so on.
“And particularly if each infected case goes on to infect, on average, more than one other person, this would be a strong warning sign that we might be in the early stages of an epidemic.”
An accompanying editorial in the BMJ, co-authored by Dr. James Rudge, concluded that while this study might not suggest that H7N9 is any closer to delivering the next pandemic, “it does provide a timely reminder of the need to remain extremely vigilant”.
According to the World Health Organization (WHO), it appears likely that the new coronavirus (NCoV) can be passed between people in close contact.
This comes after the French health ministry confirmed a second man had contracted the virus in a possible case of human-to-human transmission.
Two more people in Saudi Arabia are also reported to have died from the virus, according to health officials.
NCoV is known to cause pneumonia and sometimes kidney failure.
WHO officials have expressed concern over the clusters of cases of the new coronavirus strain and the potential for it to spread.
Since 2012, there have been 33 confirmed cases across Europe and the Middle East, with 18 deaths, according to a recent WHO update.
Cases have been detected in Saudi Arabia and Jordan and have spread to Germany, the UK and France.
“Of most concern… is the fact that the different clusters seen in multiple countries increasingly support the hypothesis that when there is close contact this novel coronavirus can transmit from person to person,” the WHO said on Sunday.
French health ministry confirmed a second man contracted the new coronavirus in a possible case of human-to-human transmission
“This pattern of person-to-person transmission has remained limited to some small clusters and so far, there is no evidence to suggest the virus has the capacity to sustain generalized transmission in communities,” the statement adds.
France’s second confirmed case was a 50-year-old man who had shared a hospital room in Valenciennes, northern France, with a 65-year-old who fell ill with the virus after returning from Dubai.
“Positive results [for the virus] have been confirmed for both patients,” the French health ministry said, adding that both men were being treated in isolation wards.
Meanwhile, the Saudi deputy minister of health said on Sunday that two more people had died from the coronavirus, bringing the number of fatalities to nine in the al-Ahsa governorate in the east of Saudi Arabia, Reuters news agency reports.
WHO officials have not yet confirmed the latest deaths.
In February, a patient died in a hospital in Birmingham, England, after three members of the same family became infected.
It is thought a family member had picked up the virus while travelling to the Middle East and Pakistan.
Novel coronavirus is from the same family of viruses as the one that caused an outbreak of Severe Acute Respiratory Syndrome (SARS) that emerged in Asia in 2003.
However, NCoV and SARS are distinct from each other, the WHO said in its statement on Sunday.
Coronavirus is known to cause respiratory infections in both humans and animals.
But it is not yet clear whether it is a mutation of an existing virus or an infection in animals that has made the jump to humans.