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Flu experts have made a mutant version of the 1918 “Spanish flu” virus that killed tens of millions of people, sparking a new debate over whether such work is too dangerous.

Yoshihiro Kawaoka of the University of Wisconsin says the experiments are important for helping scientists understand how new pandemics start, and for designing better flu vaccines.

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"Because avian influenza viruses in nature require only a few changes to adapt to humans and cause a pandemic, it is important to understand the mechanisms involved in adaptation and identify the key mutations so we can be better prepared," Kawaoka said in a statement.

Officials wearing masks and protective suits proceed to cull chickens in Hong Kong after the deadly H7N9 bird flu virus was discovered in poultry imported from mainland China. New research has created a highly deadly -- and very infectious -- version of flu virus. PHILIPPE LOPEZ / AFP - Getty Images

Previous, similar experiments by Kawaoka and other flu researchers raised such an outcry that the U.S. government also asked all flu researchers to hold off on genetically changing flu viruses until ground rules could be agreed on. The worry was that the virus could escape and accidentally cause a pandemic or, worse, that terrorists could somehow get hold of the work and use it to make a biological weapon.

The moratorium was lifted in 2012 and researchers started their work again last year.

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“The risk exists in nature already, and not doing the research is really putting us in danger,” Kawaoka said at the time.

“The worst-case scenario is the emergence of a novel avian influenza virus that exhibits high pathogenicity in human, like highly pathogenic avian H5N1 viruses, and efficient transmissibility in humans, like seasonal influenza viruses,” Kawaoka’s team wrote in the journal Cell Host & Microbe.

H1N1 has shown up again — it caused the so-called swine flu pandemic in 2009. It wasn’t nearly as deadly as other new flu viruses that cause pandemics, probably because it was a mutated form that included bits and pieces of flu viruses that had been infecting people for decades.

But other circulating influenza viruses worry experts.

H5N1 has infected 665 people in 15 countries, killing 392 of them, since 2003.

The H7N9 virus appeared a year ago in China, infecting more than 440 people and killing more than 122 of them.

Flu viruses are very prone to mutation and experts say any of the various bird flus could evolve into a strain that infects people easily, causing a pandemic that could kill millions of people.

To understand what this might look like, Kawaoka’s team first searched a giant database of known bird flu viruses, taken mostly from ducks, to find those whose genetic elements most closely matched those seen in the 1918 flu.

They found plenty. “Our study demonstrates the continued circulation of avian influenza viruses that possess 1918 virus-like proteins and may acquire 1918 virus-like properties,” they wrote.

They made a few genetic tweaks to create a version that easily passed in small, airborne droplets to infect ferrets in different cages. Ferrets are the animals that are most like humans when it comes to catching flu.

Dr. Marc Lipsitch of the Harvard School of Public Health and Dr. Alison Galvani of the Yale School of Public Health called for limits on such experiments last month.

“Potential pandemic pathogen experimentation poses a significant risk to public health, arguably the highest level of risk posed by any biomedical research,” they wrote in the Public Library of Science journal PLoS Medicine.