Progress on AstraZeneca's (NASDAQ:AZN) coronavirus vaccine candidate took a breather this week after an adverse health event triggered a pause in enrollment at trial sites around the globe. Despite the setback, on Wednesday afternoon, CEO Pascal Soriot told analysts he still expects interim results from multiple phase 3 studies of the candidate before the end of the year if those trials are allowed to continue soon.

On Tuesday, AstraZeneca issued statements describing a partial hold on phase 3 trials of AZD1222. The company is among the front-runners in the race to develop vaccines that safely teach our immune systems to recognize and attack SARS-CoV-2, the coronavirus that causes COVID-19, before it gains a foothold in the body. 

Bottles labled coronavirus vaccine.

Image source: Getty Images.

Vaccinations are so widely used that any adverse event serious enough to require the hospitalization of a participant in their clinical trials can compel researchers to put a pause on injecting them into new patients. Given the size of those late-stage trials, such events are not uncommon. Panels of independent data and safety monitors review them and usually determine them to be unrelated to the vaccine being studied, after which most trials continue without a fuss.

However, AZD1222 has the medical community being extra cautious because it uses a viral vector to do its job. Viral vectors can't infect, but some of them have been associated with severe immune system reactions in clinical-stage testing. 

In a statement issued on Thursday, AstraZeneca confirmed there had been a brief pause in the AZD1222 studies in July while a safety review took place. The volunteer whose symptoms triggered that halt was diagnosed with multiple sclerosis, a progressive neurological disorder in which patients' immune systems attack their own nervous systems.

We still don't know the diagnosis of the latest patient to pause testing of AZD1222, and AstraZeneca hasn't provided a timeline.

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