When Dr. Robert Sullivan collapsed on his treadmill three weeks after his second COVID-19 vaccine in early 2021, he fell into a "nightmare" ordeal that he said exposed glaring deficiencies in the nation's vaccine safety monitoring system.

Diagnosed with sudden onset pulmonary hypertension, the healthy and fit 49-year-old anesthesiologist from Maryland attempted to file a report through the government-run Vaccine Adverse Event Reporting System (VAERS).

But like others interviewed in a recent investigation by The BMJ, Sullivan hit barrier after barrier when trying to submit and update his report.

Almost three years later, still grappling with debilitating symptoms, Sullivan's experience highlights the systemic problems with the U.S. adverse events monitoring system run jointly by the Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA).