There's no doubt the "pause" in the trial of the vaccine being developed by AstraZeneca and Oxford University is a set-back, if only to the hopes of a planet looking for a way out of a crisis.
But it is not clear if it is just what is to be expected in the process of testing potential vaccines or something which puts a great big block on that avenue of hope.
The halting of the trial follows an "adverse" reaction among one of the participants - one of the 17,000 people taking part in the trial developed some sort of illness or ailment.
AstraZeneca described it as a "routine" pause because of "an unexplained illness" in one of the volunteers.
So is it all over - back to the drawing board?
It is not.
"Nothing to worry about - yet," according to epidemiologist Professor Adrian Esterman of the University of South Australia.
He told this paper: "All clinical trials have an independent Data Monitoring Committee. Minor adverse events are passed to the Committee on a regular (for example, monthly or three monthly) basis."
"However, serious adverse events (SAEs) are notified to the committee immediately. This could be anything from a severe allergic reaction to a stroke or even death. The committee then needs to decide whether the SAE was likely caused by the vaccine."
"This process is extremely common - there are usually several SAEs over the course of a clinical trial. However, in most cases they are deemed not to be a result of the treatment."
What will the committee want to know?
Normally, the committee is not told whether the adverse reaction was in a person given the potential vaccine or one given the fake vaccine (the placebo), but in this case the committee will be told that.
"If it's the placebo, that's great news," Dr Xavier Symons of the Plunket Institute for Bioethics at the Australian Catholic University in Canberra said.
If the person with the adverse reaction had been given the placebo, the trial continues.
This is possible, Dr Symons said, because the placebo was meant to make the recipient feel as though they had been given the vaccine. It wasn't just an innocuous, neutral substance - so it may have caused the adverse reaction.
If that is so, the trial would have been delayed by some weeks.
And if it's not the placebo?
That's a real problem.
"In the worst case scenario, the trial ends," Dr Symons said.
But it may be that the person had some sort of pre-existing condition which led to the adverse reaction.
If that is so, an assessment could be made of which people would be unsuitable for the vaccine.
Dr Symons said the vaccine could be modified - but that would be complicated and time-consuming because radical modification would mean retesting.
Nine out of 10 vaccine trials fail, Dr Symons cautioned.
What are the implications for Australia?
The potential vaccine is called "ChAdOx1 nCoV-19" - don't try and say it or ask what is stands for: what's important is that it was seen as the front-runner, so much so that the Australian government had ordered 85 million doses providing the trials were successful.
Prime Minister Scott Morrison said the whole population would get vaccinated early in the new year at a cost of $1.7 billion but there were "no guarantees".
Australia has also signed a deal worth $25 million with an American company, Becton Dickinson, for 100 million needles and syringes.
All that is now on hold, pending the outcome of the review.
At what stage were the researchers?
The potential vaccine was in a Phase III trial.
The World Health Organisation defines them as, "Phase III studies are conducted on larger populations and in different regions and countries, and are often the step right before a new treatment is approved."
The previous phases (which the Oxford research had gone through) involved tests on smaller numbers of people to evaluate safety and obvious side effects.
The Phase III for the Oxford "vaccine" involved trying it on volunteers in the UK, Brazil, South Africa, the United States and India.
The potential vaccine was given to one group and a placebo - a harmless substance - was given to a similar group to see if the "vaccine" worked on those given it and if there were dangers to it.
And then the bolt from the blue
Only on Tuesday, the Oxford researchers seemed hopeful.
"Oxford and our international partners have already vaccinated approximately 17,000 people in the first three countries selected (the UK, Brazil and South Africa), with half receiving a control vaccine," Rebecca Ashfield and Pedro Folegatti of the Jenner Institute at Oxford said.
"Most volunteers are receiving a booster vaccination one to three months after the first, as data from our phase one/two trial indicates that this strengthens the immune response - although it's not yet clear whether two doses will be necessary to protect against COVID-19."
Speed but safety
The two Oxford researchers said: "Safety, ethics and regulatory committees are speeding things up by prioritising approval processes ahead of those for other vaccines and medicines. Nevertheless, the same rigorous standards are applied to candidate COVID-19 vaccines, ensuring no corners are cut in terms of vaccine safety."