( CNN ) -- A new variation of bird flu that the WHO says has caused at least 11 deaths in China has genetic characteristics that make it well-adapted to infect people . In a report published late Thursday in the New England Journal of Medicine , samples from three patients -- all of whom died -- had mutations that have previously been shown to increase transmissibility , and to help the virus grow in a mammal 's respiratory tract . The analysis comes amid a modest but steady stream of human cases since the end of March . Saturday , China reported a 7-year-old Beijing girl is the latest person to become infected with the H7N9 flu strain , bringing the total to 44 . The strain is normally found in birds , and until last month was never known to infect people . `` The H7N9 situation is evolving very quickly , '' said Nancy Cox , director of the Influenza Division at the U.S. Centers for Disease Control and Prevention . `` One thing of concern is the pace at which we are seeing the identification of cases . '' Another death in China bird flu crisis as Hong Kong goes on alert . On a more reassuring note , investigators have found no evidence that the virus has passed directly from person to person . More than a thousand `` close contacts '' of the patients are being monitored by Chinese health officials , according to the World Health Organization . One concerning mutation , known as `` Substitution Q226L , '' was found in two of the first three victims . Past experiments have shown it to make viruses -- including the H5N1 bird flu virus -- more likely to infect ferrets , which are commonly used in flu research . The same mutation was also found in the viruses that caused the 1957 and 1968 flu pandemics . A second mutation , known as `` PB2 E627K , '' was found in all three virus samples . According to Dr. Ron Fouchier , a Dutch virologist , this mutation allows the virus to reproduce at much lower temperatures than a standard avian influenza virus . The change lets it grow in a human respiratory tract , which is cooler than the virus ' natural home : a bird 's gastrointestinal tract . In mice , Fouchier said , the mutation makes the infection as much as 1,000 times more virulent . A number of other mutations were found as well , including changes that are characteristic of viruses found in mammals . China 's bird flu mystery . `` Known normal bird viruses have to adapt substantially to infect people , but not these , '' said Fouchier , who said the changes are enough that he would no longer call the H7N9 strain `` bird flu . '' The first three patients to be identified are an 87-year-old man and a 27-year-old man from Shanghai , and a 35-year-old housewife from Anhui . The woman had visited a chicken market about a week before falling ill . The younger man was a butcher who worked in a market where live birds were sold , although he did not butcher any birds . The 87-year-old had no known exposure to live birds . All three died after suffering severe respiratory symptoms , including acute respiratory distress syndrome and eventually septic shock and multiple organ failure . In a commentary that ran with the article , Cox and Dr. Tim Uyeki , a physician with the CDC , noted that patients were not given antiviral medication until their illness became severe . Oseltamivir ( Tamiflu ) or zanamivir ( Relenza ) should be administered as soon as possible to patients with a suspected or confirmed H7N9 infection , the two wrote . Cox said it remains unclear whether the severe illnesses are typical of H7N9 infection or simply the tip of a large iceberg in which a large number of mild cases are going unnoticed . `` As surveillance has expanded , we 're also seeing individuals with milder cases , '' said Cox . `` We 're still seeing very severe disease in some cases , but overall I think it 's somewhat reassuring . '' The CDC is in the final steps of refining a diagnostic test to identify H7N9 in patients , and Cox said it should be available for distribution in a matter of days . A widely available diagnostic test would allow faster identification of patients who actually have the infection , and would also help disease detectives zero in on how people are being exposed . Work has begun on a vaccine , although Cox and others said that even if it is eventually needed , a vaccine likely wo n't be available for several months . While the overall picture is concerning , flu experts urged calm . `` I would n't say a pandemic is more likely than it was a year ago , '' Fouchier said . `` The only thing we can do as virologists right now is to point out the interesting characteristics of the virus , try to get to the bottom of this story and try to stop further infections . ''
A new human flu strain has caused 11 deaths in China . The new flu strain is responsible for 44 human cases . Experts are concerned by some of the virus mutations found .