Vietnam’s latest bird flu case has prompted an investigation into whether a 26-year-old male nurse caught it directly from an infected patient, a top scientist said on Tuesday.

But the scientist said it was not certain the case would intensify the fears of experts that the H5N1 virus, which has infected nearly all its known victims through direct contact with diseased poultry, had gained the ability to move between people.

“Our efforts focus on the human-to-human contagion of the bird flu virus in this case,” Doctor Nguyen Tran Hien, director of the Central Institute of Hygiene and Epidemiology, told Reuters.

Test results were not available yet and there was evidence the nurse, who is ill in hospital, lived in the area where poultry had been infected with the bird flu virus and that he had consumed chicken prior to his infection, Hien said.

Since the virus spread across Asia at the end of 2003, there has been one case where experts believe there was human-to-human transmission, that of a Thai woman who cradled her dying daughter in her arms for several hours.

There is no evidence yet that the virus, which has killed 47 people in Asia, has mutated into a form that could pass easily between humans with no immunity to it. Experts say if the virus did mutate in such a way, it could set off a pandemic that could kill millions of people.

Eating chicken cooked properly is safe, the experts say.

The Vietnamese nurse from the northern province of Thai Binh is being treated in a Hanoi hospital with a high fever, a symptom of the H5N1 strain that infected his patient, a 21-year-old man who caught the virus after drinking raw duck blood last month.

The nurse had provided bed care to the man, whose 14-year-old sister was also infected after coming into contact with sick poultry. State media said the brother and sister are recovering.

The Thai Binh siblings have also raised public concerns about the possibility of human-to-human transmission.

Health Minister Tran Thi Trung Chien said last week, prior to the nurse’s case, that all bird flu cases in Thai Binh, 110 km (70 miles) southeast of Hanoi, were related to slaughtering or eating poultry.

Meanwhile, the World Health Organization (WHO) said its tests on specimens from Vietnamese initially classified as negative had showed seven infections of the bird flu virus.

“WHO is awaiting further details about these cases, including outcomes,” the U.N. health agency said.

Vietnamese researchers said last week initial tests of a H5N1 vaccine on monkeys were successful, raising hopes in a country hit hard by the virus that a vaccine may be ready for human tests later this year.

The virus has killed more than 70 percent of those known to have been infected, but doctors say victims can be saved if they are diagnosed early.