The participant in AstraZeneca's /zigman2/quotes/200304487/composite AZN +5.29% /zigman2/quotes/203048482/delayed UK:AZN +6.15% Phase 3 study of its experimental COVID-19 vaccine who triggered the clinical trial's hold reportedly was diagnosed with transverse myelitis, which is a spinal disorder potentially associated with vaccines, according to RBC Capital Markets' Brian Abrahams. AstraZeneca's stock was up 0.9% in premarket trading on Thursday. The New York Times first reported the diagnosis of transverse myelitis, which has not been confirmed by AstraZeneca; however, the company has confirmed it halted the trial due to a "potentially unexplained illness." Abrahams wrote in a note on Tuesday to investors that instances of vaccine-related transverse myelitis are rare, though there are 119 cases recognized by the Food and Drug Administration between 1985 and 2017. "It makes sense that the [adverse event] will be fully investigated prior to continuation of vaccine administration," he wrote. "It also highlights the importance of following volunteers for weeks to months post-dosing, as if this side effect were actually related to this or other COVID-19 vaccinations." Shares of AstraZeneca, which is developing the vaccine with the University of Oxford, are up 7.6% so far this year. The S&P 500 /zigman2/quotes/210599714/realtime SPX -1.70% has gained 5.2%.