Shares of the drugmaker fell $2.99, or 5.5%, to $51.71 in late trading.
The trial was halted after “a suspected serious adverse reaction in a participant in the United Kingdom,” Stat reported. AstraZeneca is developing the vaccine candidate with the University of Oxford.
The company said in a statement that a “standard review process triggered a pause to vaccination to allow review of safety data,” according to the report. It was not clear whether the company itself had ordered the pause, or if a regulatory agency had.
Late-phase trials of the vaccine began in the U.S. last month, though studies are also being conducted in the UK, Brazil and South Africa, according to the report.
COVID-19 vaccine trials are being closely watched as the best hope for ending the global pandemic that has killed nearly 900,000, sickened more than 27 million and caused economic turmoil worldwide. Nearly 200,000 of the deaths have been recorded in the U.S. amid a disorganized and highly politicized response to the disease.
President Donald Trump has staked much of his re-election hopes on the accelerated approval of a vaccine, prompting fears that safety guidelines would be ignored in the rush to announce a success ahead of the Nov. 3 election.
Earlier Tuesday, CEOs of nine drug manufacturers released a signed pledge to adhere to proper safety guidelines as they pursue development of a vaccine.
The companies included AstraZeneca, BioNTech (BNTX) - Get Report, GlaxoSmithKline plc (GSK) - Get Report, Johnson & Johnson (JNJ) - Get Report, Merck (MRK) - Get Report, Moderna (MRNA) - Get Report, Novavax, Inc. (NVAX) - Get Report, Pfizer Inc. (PFE) - Get Report, and Sanofi (SNY) - Get Report.