The disclosure that AstraZeneca and Oxford have suspended their phase three vaccine trials following a suspected case of transverse myelitis - a rare neurological condition - in a British female participant is bad news, but it may yet prove to be just a bump in the road.
The pause will delay the vaccine's progress, making the possibility of it receiving an emergency licence this year much less likely. But experts expect the trial will almost certainly continue to allow further data to be gathered and assessed.
The scientific community was out in force yesterday to stress the positives. The suspension of the trial, they said, demonstrated that patient safety was paramount and that the complex system of regulatory checks and balances was working as it should.
"It's always a concern if someone in the trial develops a serious adverse reaction... but if we had five or six cases I would be much more alarmed," said Professor Beate Kampmann, director of the vaccine centre at the London School of Hygiene and Tropical Medicine.
"Whether this reaction is related to the vaccine under trial itself, or is unrelated, now has to be established. But to think this vaccine is now doomed is completely disproportionate."
Yet the incident raises cause for concern and stock prices in AstraZeneca tumbled after the news broke.
Ever since President Gerald Ford rushed through the licensing of a novel flu vaccine in 1976, fears of vaccines prompting dangerous neurological side effects have stalked the industry.
Ford acted against scientific advice to license the mass distribution of a novel HIN1 shot in the US after an outbreak of a new strain of influenza at the Fort Dix military base in New Jersey.
There was no pandemic, but the vaccine caused Guillain-Barré syndrome - a neurological condition that can paralyse - in some 450 of the 45 million people who received the jab. More than 30 died, sparking a scandal that underpins much of today's safety regulations.
Pascal Soriot, AstraZeneca's chief executive, confirmed yesterday that the Oxford vaccine trial was suspended after a woman was admitted to hospital with symptoms consistent with transverse myelitis.
She is recovering and was due to leave hospital today.
Soriot added that the trial had also been paused in July after a different participant experienced neurological symptoms.
However, these were found to be unrelated to the vaccine.
Transverse myelitis can cause paralysis, sensory problems and bladder and bowel dysfunction. Most people recover, but severe attacks can cause lasting and serious disabilities.
Experts say it will be all but impossible for AstraZeneca to know if the adverse event was caused by the vaccine.
Instead, they will look at the background incidence of transverse myelitis in the population to understand the probability of it being a random event. It is likely the trial will then continue with additional protocols in place.
The pausing of the trial is a blow, not least because the Oxford vaccine is one of the most promising available outside of China. The jab is considered a leader not just because it has made substantial progress, but because it is keenly priced and relatively easy to make.
Sir Patrick Vallance, the British Government's Chief Scientific Adviser, said there is a "reasonable chance" a coronavirus vaccine will become available next year.
He told a press conference at No 10 that of the 200 projects around the world, six were in the final stages of testing. "Many of these vaccinations are showing the right immune response. That is a very important step along the way of making a vaccine," he said.