Flu researchers plan to resume controversial experiments into bird flu transmission that were voluntarily suspended over biosecurity fears.

Last year, scientists agreed to a moratorium on changing the H5N1 avian flu virus in the lab to make it transmissible between ferrets through respiratory droplets. They want to understand how the bird virus could spread through the air to mammals such as humans.

Ron Fouchier said his bird flu experiments in ferrets could resume in weeks. (Courtesy of Ron Fouchier/Science )

By learning more about how the virus could evolve the ability naturally, scientists hope to contain its potential to cause a pandemic in humans.

The H5N1 virus occasionally infects individuals, but doesn't spread from person to person. Scientists worldwide are watching closely to see if the virus mutates and acquires that ability.

Ron Fouchier, a Dutch virologist whose laboratory at Erasmus Medical Centre in Rotterdam, told reporters Wednesday that his ferret experiments in the Netherlands with new strains of H5N1 that could spread through the air could be up and running in weeks.

"Acknowledging that the aims of the voluntary moratorium have been met in some countries and are close to being met in others, we declare an end to the voluntary moratorium on avian flu transmission studies," Fouchier and 39 other scientists wrote in a letter published by the journals Science and Nature.

The experiments will be conducted outside the U.S., where officials are completing a framework to permit such work by researchers funded or based in that country. The first countries that could be involved include the Netherlands and China.

Biosecurity in Winnipeg

Gary Kobinger, chief of the special pathogens laboratory at the Public Health Agency of Canada's National Microbiology Laboratory in Winnipeg, signed the letter. Kobinger said it's important for the international experiments to continue.

"This is the first window open that we have where we can test the vaccine ahead of time, we can test the drugs ahead of time, we can study and understand exactly how is this virus going to present itself, so when it does, we can detect it right away," said Kobinger.

Scientists at the Winnipeg lab, a top biosecurity facility, work with samples of H5N1 from infected humans and have used the 1918 pandemic flu strain.

Fouchier has U.S. funding but said he'd use his European Union grants for the work, which has been approved by governments and institutions there.

An editorial in Nature accompanying the letter said the lifting of the moratorium by the researchers must not be seen as the end of the debate.

Responsibility and prudence

"It is clear that the immediate practical applications of gain-of-function flu research remain largely hypothetical, and that its true value lies in long-term fundamental research to improve understanding of the transmissibility and pathogenicity of the virus," the editors wrote.

"That makes it even more incumbent on researchers and authorities to exercise the greatest responsibility and prudence."

They said the proposed U.S. framework spells out criteria that need to be met before funding, such as whether safer approaches exist to answer the same questions.

Guidelines from the World Health Organization outline the type of facilities where the experiments can take place and encourage a culture of safety, a move the signatories supported.

Kobinger also welcomed critics of the experiments to air their concerns about publicly funded research and contribute to the ongoing discussions.

With files from CBC's Amina Zafar