A pause in the trial of one of the world's most promising COVID-19 vaccines shouldn't delay the process too long, an expert says.
In fact it's a sign the researchers are taking health and safety seriously, says Oksana Pyzik, public health expert at University College London.
The AstraZeneca-Oxford University vaccine trial is currently in its third phase, involving 30,000 people in the US and UK. The trial was halted after one fell sick with an undiagnosed illness.
The New York Times reports it's transverse myelitis - an inflammatory syndrome often sparked by viral infections that affects the spinal cord. It's quite rare - affecting between just one and eight people per million every year - hence the concern that it's popped up in a recipient of the experimental vaccine.
"This particular adverse event may or may not be related to the vaccine," Pyzik told The AM Show on Thursday.
"They may have an underlying medical condition or another element that has caused this, but it needs to be investigated safely. It's been put on pause, but this is very common. It happens frequently in clinical trials... due process and not rushing or skipping any steps, is critical to get public trust."
A rival Russian-made vaccine, on the other hand, was rushed into phase 3 trials despite no data being published for international scientists to review. After the phase 1 and 2 data was published earlier this week, an international group of experts penned an open letter expressing concern, saying the results were "highly unlikely".
Transverse myelitis occasionally crops up in vaccine trials, as some vaccines use live - but weakened - versions of the virus they're meant to protect against. The AstraZeneca-Oxford University vaccine doesn't.
"It wouldn't be possible for the vaccine to cause a COVID-19 infection, which could then spark the myelitis," researchers Nigel William Crawford and Jim Buttery wrote in a piece for website The Conversation.
Also, some of the participants in the trial aren't getting the experimental vaccine at all - it's what's known as a double-blind trial, meaning the researchers typically don't know which participants got the vaccine and which didn't until after the experiment is over. It could be the person who had the adverse reaction got an already proven-safe meningococcal vaccine instead.
The third phase of the trial also includes older people, who are more likely to have adverse reactions than the younger people who typically take part in phases 1 and 2.
AstraZeneca said it was "working to expedite the review of the single event to minimise any potential impact on the trial timeline".
Pyzik said there are several vaccine candidates in development around the world, and since each only has a small chance of success, a few weeks of delay in this particular trial wouldn't make much difference, if any, to how soon the world might have a COVID-19 vaccine.
"Generally speaking there is only a 10 percent chance of any vaccine to cross that finish line, but we are boosting our chances by broadening the competition and introducing novel forms of development. So there are reasons to be optimistic - this shouldn't be a major setback in the sense of the timeline.
"This happens frequently in trials, and it's a good thing the safety measures are being followed. There are several other vaccines including Novovax that will probably be entering phase III - the biggest and most expensive part of the trial - soon.
"All of that is reason to have some optimism while we continue with the standard public health measures that are required to contain and suppress the virus."
Buttery and Crawford said usually, a halt in a trial wouldn't make headlines - it's just the need for a COVID-19 vaccine is so great.
"There's perhaps never been so much attention on a single clinical trial as there is on the trial of this and other potential COVID-19 vaccines."