The front-runner in the race to develop a coronavirus vaccine recently hit a stumbling block. Around the world, phase 3 trials with AZD1222, a SARS-CoV-2 vaccine candidate from AstraZeneca (NASDAQ:AZN), have stopped enrolling new patients after a volunteer reported an adverse event.

Can the AZD1222 program recover from this setback? The short answer is yes, but this story isn't finished. Let's take a closer look at what happened so we know what to expect in the weeks and months ahead.

Person in face mask holding coronavirus tube.

Image source: Getty Images.

What happened

Late in the afternoon on Sept. 8, Stat received the sort of statement from AstraZeneca that was bound to appear from one of the coronavirus vaccine development front-runners sooner or later. An unexplained illness triggered a pause in enrollment of a phase 3 trial while independent data monitors look for any signs that suggest AZD1222, a vaccine candidate AstraZeneca licensed from Oxford University, was to blame.

According to people familiar with the matter, the trial participant enrolled in the U.K. and is expected to recover. We don't know anything yet about the nature of the adverse event that led AstraZeneca to pause enrollment into phase 3 studies in the U.K. and around the globe.

Reasons to relax

Protocols vary from study to study, but it generally doesn't take much to pause clinical trial enrollment when an experimental new drug is involved. So far, thousands of patients have received injections of AZD1222, and this is the first time AstraZeneca's issued a statement concerning a trial-pausing adverse event.

AstraZeneca said it voluntarily paused vaccination to give the trial's independent data monitors time to review available safety data. At the moment, we don't know if the patient who triggered the enrollment pause had been randomized to receive AZD1222 or a placebo.

Reasons to worry

The AZD1222 program is trying to do something that hasn't been done before. Instead of directly injecting something that resembles SARS-CoV-2, the virus that causes COVID-19, AZD1222 employs an adenovirus isolated from chimpanzees as a delivery vehicle for genetic material. Human cells use the genes to produce proteins that mimic those found on SARS-CoV-2's surface, which, in turn, teach the immune system to immediately recognize the real thing.

Using viral vectors to transfer genetic material isn't anything new, but approved treatments that use this technique are currently limited to diseases that are both severe and rare. Viral vectors have no means of self-reproduction, but their temporary appearance can spur some immune systems to overreact.

Physician with clipboard in a ICU.

Image source: Getty Images.

This is the second time an adverse event has paused a phase 3 study with AZD1222. In July, a volunteer developed neurological symptoms and was later diagnosed with multiple sclerosis.

We still don't know the nature of the trial-pausing event that inspired AstraZeneca to issue statements this time around. According to AstraZeneca, the latest patient to experience an adverse event hasn't received a final diagnosis yet. If it turns out the latest adverse event involved neurological symptoms, it could be lights out for the AZD1222 program.

What to expect

AstraZeneca hasn't made public the clinical trial protocols that determine which adverse events trigger a pause in enrollment. Complete trial protocols are rarely disclosed, but AstraZeneca and its peers probably should be more transparent about development issues regarding their taxpayer-funded vaccine programs.  

We're all hoping events that paused clinical trials with AZD1222 are entirely unrelated to the vaccine, but it's important to remember most vaccine candidates that begin clinical trials eventually fizzle out without earning approval. If AstraZeneca isn't more forthcoming soon, go ahead and assume the worst. Odds are you'll be right.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.