An officer who witnessed the incident, said it seemed to happen “all at once.”
“It seemed like it happened over the course of just a few minutes,” he said. “Suddenly we had several people down on the ground experiencing some type of cardiac arrest or medical episode and so we immediately started doing CPR and moving people.”. https://www.click2houston.com/news/local/2021/11/06/hfd-responding-to-reports-of-multiple-injuries-at-astroworld-festival/
Fentanyl reaction to vaxx seems like most logical theory to me.