The breakthrough raised concerns among health and security experts who warned it could prompt a pandemic, or ease a terrorist attack.
On Wednesday, 40 flu researchers from Europe, Asia and the United States published an open letter defending their decision to resume the studies.
"Because H5N1 virus-transmission studies are essential for pandemic preparedness and understanding the adaptation of influenza viruses to mammals, research who have approval from their governments and institutions … have a public-health responsibility to resume this important work," the scientists wrote.
"We fully acknowledge that this research … is not without risk," they continued. "However, because the risk exists in nature that an H5N1 virus capable of transmission in mammals may emerge, the benefits of this work outweigh the risks."
The letter was published in the journals Science and Nature.