The summit organisers were similarly critical in a highly anticipated consensus statement issued on the event's last day.
"At this summit we heard an unexpected and deeply disturbing claim that human embryos had been edited and implanted, resulting in a pregnancy and the birth of twins," said the summit's organising committee, which called for independent verification of He's claims that have so far not been published in a peer-reviewed journal.
"Even if the modifications are verified, the procedure was irresponsible and failed to conform with international norms," the organisers said in the summit's consensus statement that is usually seen as setting the tone and direction for the fast-changing field.
The committee, representing leading researchers from the United States, Britain and Hong Kong, did not call for an outright ban on gene editing.
Instead, it acknowledged that the field was moving toward a future where the procedures would be widely researched in clinical trials, and that researchers needed a rigorous framework to set ethical standards and guidelines. But in the meantime, the panel called for a halt.
"The organising committee concludes that the scientific understanding and technical requirements for clinical practice remain too uncertain and the risks too great to permit clinical trials of germ line editing at this time," the closing statement said.
He's work was widely criticized this week by peer researchers and ethicists as a rogue demonstration of a gene editing tool called CRISPR-Cas9, which has opened up a world of new possibilities in biomedical research in recent years. Proponents foresee a time when lethal genetic diseases with no treatments could be eradicated, while critics fear that the technology might be used for casual genetic enhancements, to tweak traits like intelligence or height.
But until this week, such debates were largely theoretical, because no one was known to have established a pregnancy from a genetically-edited human embryo. He's claimed experiment was a rude awakening for scientists at the summit, and an urgent reminder that discussions about how to responsibly use a technology that could reshape the health and character of future generations might not be enough.
"The need for development of binding international consensus on setting limits for this kind of research . . . has never been more apparent. Without such limits, the world will face the serious risk of a deluge of similarly ill-considered and unethical projects," Francis Collins, director of the National Institutes of Health, said in a statement. "Should such epic scientific misadventures proceed, a technology with enormous promise for prevention and treatment of disease will be overshadowed by justifiable public outrage, fear, and disgust."
Many countries outlaw such experiments, and in the United States, using genetically altered human embryos for reproductive purposes is effectively banned by law.
After word first leaked of He's project through news reports, scientists criticized the effort as irresponsible and premature. Investigations were opened into He's work by China's Southern University of Science and Technology, which has said the university was not informed of the experiment. He appeared before a packed audience in person and online to defend his work at the summit on Wednesday, but the presentation was far from convincing to experts.
Summit organisers concluded that He's work was deeply problematic. "Its flaws include an inadequate medical indication, a poorly designed study protocol, a failure to meet ethical standards for protecting the welfare of research subjects, and a lack of transparency in the development, review and conduct of the clinical procedures," they said.
Outside scientists and ethicists slammed the experiment for being medically unnecessary, because the babies wouldn't have been born infected with the virus, which can also be prevented with existing, low-risk interventions. They questioned whether the effort had even succeeded at its own goal of making both girls immune to HIV after seeing He's data. And they warned the research had not ruled out potentially harmful unintended effects that could afflict the twins and now spread through the human lineage if they have children.
"Having listened to Dr. He, I can only conclude that this was misguided, premature, unnecessary and largely useless," said R. Alta Charo, a bioethicist at the University of Wisconsin-Madison.
For years, leading scientists have avoided advocating a ban on gene-editing technology for human reproduction, instead favoring a cautionary approach that such research should not proceed until certain conditions are met.
At the last meeting of the summit in 2015, scientists concluded by saying that it would be "irresponsible" to proceed until the safety concerns had been thoroughly vetted and a societal consensus had developed. But a report two years later by the National Academies of Sciences said that genome edits that could be inherited "might be permitted" if, for example, there were transparency and an unmet need, among other criteria. A report this summer by the Nuffield Council on Bioethics in the United Kingdom concluded that gene editing to influence future generations "could be ethically acceptable in some circumstances."
Those recommendations suggested the technology could be useful, which may have emboldened He. When he presented his research on Wednesday, he said he was "proud" of his work.
"My original thinking was based on the survey of the United States . . . or the British ethics statement or the Chinese study that gave us the signal that the majority of the public is supporting the use of human genome editing for treatment, including HIV prevention," He said.
Mathew Porteus, a paediatric stem cell scientist at Stanford University, said that in February, He told Porteus about his animal studies and an open trial in humans.
"I told him that it was irresponsible and reckless to proceed for many reasons and that he needed to discuss his plans with senior authorities in China before proceeding any further," Porteus said.
Several scientists said that a ban would be premature, but added that the technology was not ready to be used. Others said a clear message needed to be sent.
"My sense would be, given the circumstances, that anything short of a call for moratorium would be insufficient," said Paul Knoepfler, a stem cell biologist at the University of California at Davis. "This has indicated that almost anyone could try this; this guy's not a physician, he's a physicist . . . There seems to be need for more clarity."
