Less than a week after investors learned that AstraZeneca (AZN 0.14%) had paused the clinical trials testing its coronavirus vaccine, AZD1222, the U.K.'s Medicines Health Regulatory Authority (MHRA) determined it was safe for the company to restart the studies.

The pause was triggered by an adverse event -- clinical-trial speak for a potential side effect -- in one participant in the clinical trial. Pauses, which are fairly common in large clinical trials, give companies time to determine if the adverse event was caused by the vaccine. An independent committee for the U.K. studies reviewed the data and recommended to the MHRA that the clinical trials were safe to resume.

Hand holding a syringe near a patient's shoulder.

Image source: Getty Images.

About 18,000 people have been vaccinated with AZD1222, which is also being tested in phase 3 clinical trials in Brazil, South Africa, and the U.S. AstraZeneca didn't say when the clinical trials outside of the U.K. would be restarted.

AstraZeneca also didn't disclose publicly what the illness was, but multiple sources have reported that the company's CEO Pascal Soriot said on a call organized by the investment bank J.P. Morgan that the patient experienced serious neurological symptoms that were consistent with transverse myelitis, a spinal inflammatory disorder.

Shares of AstraZeneca were up just 0.7% in late-afternoon trading on the news, but the drugmaker had already regained most of the decline its shares suffered after the clinical-trial pause was disclosed.