Vanity Fair and other news sources have cited multiple unnamed sources from within the US State Department claiming that three Wuhan Institute of Virology researchers supposedly in engaged in "gain of function" studies had fallen ill in November 2019 with symptoms similar to Covid-19:
In November, that lead turned up classified information that was “absolutely arresting and shocking,” said a former State Department official. Three researchers at the Wuhan Institute of Virology, all connected with gain-of-function research on coronaviruses, had fallen ill in November 2019 and appeared to have visited the hospital with symptoms similar to COVID-19, three government officials told Vanity Fair.
While it is not clear what had sickened them, “these were not the janitors,” said the former State Department official. “They were active researchers. The dates were among the absolute most arresting part of the picture, because they are smack where they would be if this was the origin.” The reaction inside the State Department was, “Holy shit,” one former senior official recalled. “We should probably tell our bosses.” The investigation roared back to life.
Is any of this verifiable by the general public? Who are these three researchers? Were they ill on the dates claimed with the symptoms claimed, etc.?
There are some rebuttals from China's Ministry of Foreign affairs, but they are rather unspecific and simply accuse the US of spreading anti-Chinese propaganda. The most specific of these is probably this quote on WebMD:
“I’ve read it. It’s a complete lie,” Yuan Zhiming, PhD, director of the biosafety lab, told Global Times, a state-run news outlet.
“Those claims are groundless,” he said. “The lab has not been aware of this situation, and I don’t even know where such information came from.”
It's not too clear whether that is in response to the January 15 factsheet from Trump's administration (which was a bit vague) or the more recent, slightly more detailed report published by the Wall Street Journal and other outlets in May.