Wall Street analysts lay out what the unexpected pause in AstraZeneca's coronavirus vaccine trial could mean for the race to halt the pandemic

• AstraZeneca has put its coronavirus vaccine trial on hold after a trial participant had a suspected adverse event.
• The voluntary hold could set back the late-stage vaccine trial by weeks at best, Wall Street analysts wrote on Wednesday.
• Should AstraZeneca find the reaction was caused by the vaccine, it could put the vaccine trial on hold for longer, if not permanently.
• Shares of AstraZeneca declined on Wednesday, while rival vaccine makers Moderna and Pfizer were up.

There's been a setback in the race to develop a vaccine that prevents the novel coronavirus.

AstraZeneca, the UK drugmaker leading the race to create a coronavirus vaccine, on Wednesday said it had placed a hold on its late stage trial after a participant had a suspected adverse event. The drugmaker's statement followed a report from Stat News that the trial had been paused.

"This is a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials, while it is investigated, ensuring we maintain the integrity of the trials," AstraZeneca said in a statement. The company didn't say how long the hold would last.

Citing a person familiar with the situation, The New York Times reported that the participant in the late-stage trial had received a diagnosis of transverse myelitis, an inflammatory condition that impacts the spinal cord and is often associated with viral infections. It's still unclear whether the condition is linked to the vaccine.

Read more: There are 176 coronavirus vaccines in the works. Here's how the top drugmakers see the race for a cure playing out in 2020 and 2021 and when the first shots might be available.

It's also unclear what the ramifications will be for other vaccines under development, though investors sent the stocks of AstraZeneca's rivals Pfizer and Moderna higher on Wednesday, betting any delay for AstraZeneca would work to their advantage.

Dr. Anthony Fauci, the US's top infectious-disease doctor, said the trial pause is a sign that drugmakers are proceeding with appropriate caution as they work to counter the pandemic.

"It's really one of the safety valves that you have on clinical trials such as this," Fauci, the director of the National Institute of Allergy and Infectious Disease told CBS News on Wednesday. "So it's unfortunate that it happened, hopefully they'll work it out and be able to proceed along with the remainder of the trial. But you don't know. They need to investigate it further."

AstraZeneca's trial pause could lead to weeks of delay in figuring out whether the vaccine works, at best, Wall Street analysts said in notes Wednesday morning.

"AstraZeneca has said the pause is a routine action which has to happen whenever there is a potentially unexplained illness in a trial," Jefferies analyst Peter Welford wrote in a note Wednesday. "It added that it is working to expedite the review of the single event to minimise any potential impact on the trial timeline."

SVB Leerink analyst Dr. Andrew Berens noted that if the adverse event proves to be transverse myelitis, the study could be postponed for quite some time.

"If this is the case, AstraZeneca's vaccine study may be postponed by weeks to months, as the safety database is scrutinized and trial protocols revised," Berens wrote in a note Wednesday.

The trial hold could have a range of implications for the vaccine race

Berens outlined four scenarios that could play out once AstraZeneca learns more about the unexplained illness related to the trial.

• Under the first scenario, AstraZeneca finds that the potential adverse event isn't transverse myelitis, or that the transverse myelitis diagnosis isn't related to the vaccine. Even if that's the case, it could take weeks if not months to prove the vaccine doesn't cause the reaction, Berens wrote. On the whole, this scenario doesn't have much of an impact on the other vaccines being tested, and it could mean that the AstraZeneca vaccine goes back on track.
• In a second scenario, AstraZeneca might find that the transverse myelitis diagnosed in a participant is related to its vaccine. AstraZeneca's using a vaccine platform different what Pfizer and Moderna are using, which might mean that the implications are limited to just the one vaccine. Even so, it could mean the end of the line for AstraZeneca's vaccine.
• In a third scenario, the potential adverse event could have farther-reaching implications. If the reaction is related to the biology of the virus itself and the way the vaccine interacts with it, it could spell trouble for other vaccine frontrunners.
• In a fourth scenario, if the potential adverse event is related to the immune response to the novel coronavirus brought on by the vaccine, it could have implications for vaccine development as a whole.

Morgan Stanley analysts Matthew Harrison and David Risinger said in a note Wednesday that the only type of adverse event that might impact all COVID-19 vaccine platforms would be antibody-dependent enhancement, in which the patients who are vaccinated have more serious cases of COVID-19 than those who aren't vaccinated.

"We do not believe that the event described by AstraZeneca is likely the cause of ADE," the analysts wrote.

To be sure, it'll take a while to ge answers to those questions.

"Resolving this question, and proving the negative, could take months for this program, and has implications for the development strategy (but not the development risk) for all the other COVID vaccines in pivotal trials," Leerink analyst Geoffrey Porges wrote in a note Wednesday.