We should expect to get serological data from excess Chinese mortality before December then. Assuming they agree to good back sampling. They have biopsies like any public health system.

The Chinese government has been pushing the narrative that their reported case counts exactly describe reality and there were no widespread undetected infections. I wouldn't rely on any evidence that contradicts that ever officially seeing the light of day, given the heavy central control over coronavirus-related research and the sensitivity of the subject. Supposedly there were antibody testing results from Wuhan leaked that showed a 5% infection rate there but I don't think anyone has been able to confirm this.