Scientists who created a mutant virus to explore a key aspect of influenza published their research Wednesday after a four-month storm that brewed fears of bioterrorism and accusations of censorship.

The controversy began in December when teams in the United States and the Netherlands separately said they had engineered a hybrid virus in high-security labs.

Their goal was to understand how a highly lethal strain of flu which spreads among birds but is hard to transmit to mammals could mutate into a variant that is contagious among humans.

A 23-member expert panel that advises the US government called for manuscript changes before the work could be published in a journal, the traditional arena for displaying and discussing scientific work.

It feared that full disclosure could help a rogue state or bioterror group make a virus against which no-one would be immune.

But some scientists lashed the recommendation, saying it was an attempt to censor or stifle scientific discourse.

Two journals put the papers on hold while they consulted the researchers and the panel, the National Science Advisory Board for Biosecurity (NSABB).

On Wednesday, the British journal Nature finally published one of the studies, conducted by a team led by Yoshihiro Kawaoka at the University of Wisconsin.

"The essential scientific elements (in the original manuscript) were unchanged," the journal said, adding it was publishing the paper after receiving "several independent pieces of biosecurity advice".

Kawaoka's team delved into the H5N1 strain of avian flu, which caused a health scare in Hong Kong in 1997 and still surfaces sporadically today.

H5N1 spreads easily among poultry and wild birds but is hard to transmit to humans. When it does, it is brutal, killing more than one infected person in two.

The team took a key gene, known as haemagluttinin or HA, from the H5N1 virus and added a mutation that made it more compatible with human respiratory cells.

They then took a strain of H1N1 flu -- the virus that caused a pandemic among humans in 2009 but proved to be no more lethal than ordinary seasonal flu -- and replaced its HA gene with the engineered one.

The next step was to test the "H5/H1 hybrid" on six ferrets, a mammal deemed an excellent model for testing flu because its respiratory system is so similar to that of humans.

The infected ferrets passed on the virus to others in respiratory droplets, thus proving that the new virus could be spread through coughs and sneezes.

But none of the animals died, something that remains to be explained, said the researchers.

The findings shed light on the genetic borrowings that help a virus gain in contagiousness, they said. This risk is all too present in nature, especially in pigs, which can mix avian, human and porcine viruses.

The work will help alert health watchdogs to emerging viral threats and provide vaccine engineers with potential targets, they argued.

Nature showed journalists a report from "a bio-defence agency outside the US", which it declined to name, that said the benefits of publication outweighed the risks.

"This information could be used by an aggressor and shows one of the building blocks for the development of a potential BW [biowarfare] weapon," the report said.

"[Such skill] is a demanding capability, probably beyond the capacity of the majority of those groupings of concern," it said.

"On the other hand, not publishing this information would slow, or even block, the development of a vaccine against a virus that still has the potential to mutate naturally to a pandemic form, which could cause huge numbers of fatalities worldwide."

Touching on the tension between freedom of expression and scientific responsibility, Nature said it was "desirable" to have a forum such as NSABB but in this case the panel had over-reacted.

"There are justified concerns among the research community about the NSABB's processes, and these processes should be reviewed."

The other paper, intended for the US journal Science, is written by Ron Fouchier of the Erasmus Medical Centre in Rotterdam.

The American Association for the Advancement of Science (AAAS), which publishes the journal, said Fouchier's study was undergoing peer review -- the traditional scrutiny in scientific publications.

"We had originally hoped, as a public service, to be able to publish Dr. Fouchier's paper simultaneously with the similar research by Dr. Kawaoka. But appropriate review and editing of the manuscript is the primary goal," said AAAS spokeswoman Ginger Pinholster.