A trial participant for a promising Covid-19 vaccine suffered from mysterious neurological symptoms including limb weakness or "changed sensation", new research papers show.
The University of Oxford and partner AstraZeneca paused the trial of the vaccine over the scare last week.
However, the trial has since resumed and researchers say the symptoms, which arose in a participant in the UK, were likely not linked to the shot.
"After independent review, these illnesses were either considered unlikely to be associated with the vaccine or there was insufficient evidence to say for certain that the illnesses were or were not related to the vaccine," a participant information sheet posted online by Oxford reads.
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"In each of these cases, after considering the information, the independent reviewers recommended that vaccinations should continue."
Researchers have noticed side effects from the vaccine before, but they have been listed as mild or moderate.
A Phase 1/2 study published in July reported that about 60 per cent of 1000 participants given the vaccine experienced side effects.
All of the side effects, which included fever, headaches, muscle pain and injection site reactions, were deemed mild or moderate. All of the side effects reported also subsided during the course of the study.
Trial holds are not uncommon, but last week's setback is a blow to worldwide hopes for a shot to be ready in the coming months, as the AstraZeneca shot was considered by many – including the World Health Organisation (WHO) – to be the leading candidate worldwide.
Researchers had hoped to know whether the vaccine worked and was safe by year-end, but that now looks increasingly unlikely.