AstraZeneca Plc Chief Executive Officer Pascal Soriot said the coronavirus vaccine the company is developing with the University of Oxford could still be ready before the end of the year after pausing its trials due to a possible serious neurological problem in one participant.

Speaking at an online conference Thursday, Soriot sought to reassure investors after the company and its partner confirmed earlier this week that they had temporarily stopped giving people the experimental shot.

The trial was halted after a person in the UK who was participating in it got sick, triggering a review of safety data. Though such interruptions are common in vaccine studies, the drugmaker and its boss are facing questions about what exactly caused the issue and whether it could be related to the shot.

“What we have here is a special set of circumstances where the whole world becomes involved in the conduct of a clinical trial,” Soriot said in his first public comments since the trial was halted. The decision on whether to resume the study is in the hands of a group of independent experts working to understand whether the patient’s illness was a coincidence or a result of the vaccine. “The reality is we all have to be very patient and see how it unfolds,” he said

Astra shares slipped 0.6% in London.

It’s still not clear whether the participant has a condition called transverse myelitis, a suspected diagnosis, Soriot said. The CEO, speaking at a Tortoise Media event Thursday, said he can’t evaluate the length of the trial pause. When tests are complete the physicians will share the data with the safety committee, he said. The development could slow Astra down.

“However long it takes, it is going to introduce some delay in that program,” Richard Hatchett, head of the Coalition for Epidemic Preparedness Innovations, an Oslo-based group working to accelerate experimental inoculations, said in an interview. “Whether that delay is just a week or whether it’s several weeks, or even several months, remains to be seen.”

The pause highlights the importance of rigorous final-stage trials to pick up on potential side effects or rare events, according to Hatchett. It also shows the need to have a diversified portfolio of Covid-19 vaccines under development, a strategy that CEPI is using with its partners, he said.

“It points to the fact that vaccine development is tricky and unpredictable, and that things that look like they are going along very well can encounter difficulties,” he said.

US National Institutes of Health Director Francis Collins told a Senate committee Wednesday the trial had been halted due to a “spinal cord problem.” AstraZeneca is one of several drug companies taking part in the US government’s Operation Warp Speed program to accelerate the development of a coronavirus vaccine.