I look to the Soviet Union for answers.

When Kyshtym happened, there was documentation, there was a paper trail. Nobody really saw it outside of the circle of trust because the Soviets managed to keep things on the DL but there was paper. When Chernobyl happened? It got out so we have documentation (trustworthy or not) by the minute. You can continue on with the space program: we've got horrifically bad footage of Nedelin, even though it was denied by the Soviets/Russians until 1995.

Sverdlovsk, on the other hand? We have first person accounts of how the release happened and the Russians still denied it happened. Aralsk? 50,000 vaccinations but nothing actually happened there. We don't even know that much about the Rajneeshee attack even though Ma Anand Sheela entered an Alford Plea and went to jail for 29 months.

Ken Alibek pointed out that one of the principle advantages of a biological warfare program is it's extremely easy to hide. Top US officials legit toured Vektor and found nothing despite the fact that the Soviets had produced 20,000 tons of anthrax spores there the previous year and were in production less than two months earlier.

We're never going to know. There will be all sorts of circumstantial evidence, but every politician will describe it as circumstantial evidence, and every businessman will continue to buy cheap chinese shit made by people with a legitimate fear that if their social credit drops too low Xi will harvest their organs.