New Delhi, Sep 9 : The testing of Covid-19 vaccine being developed by AstraZeneca and University of Oxford has been put on hold due to a serious adverse reaction in a participant in the UK. Stat News reported that a large, Phase 3 study testing a Covid-19 vaccine being developed by AstraZeneca and the University of Oxford at dozens of sites across the US has been put on hold due to a suspected serious adverse reaction in a participant in the UK. A spokesperson for AstraZeneca, a frontrunner in the race for a Covid-19 vaccine, said that the company's "standard review process triggered a pause to vaccination to allow review of safety data". In a follow up statement, AstraZeneca said it initiated the study hold. The nature of the adverse reaction and when it happened were not immediately known, though the participant is expected to recover, Stat News reported. The spokesperson described the pause as "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". The spokesperson also said that the company is "working to expedite the review of the single event to minimize any potential impact on the trial timeline". An individual familiar with the development said researchers had been told the hold was placed on the trial out of "an abundance of caution". A second individual familiar with the matter, who also spoke on condition of anonymity, said the finding is having an impact on other AstraZeneca vaccine trials underway -- as well as on the clinical trials being conducted by other vaccine manufacturers, according to the Stat News report. AstraZeneca only began its Phase 3 trial in the US in late August. The US trial is currently taking place at 62 sites across the country, according to clinicaltrials.gov, a government registry, though some have not yet started enrolling participants. Phase 2/3 trials were previously started in the UK, Brazil, and South Africa. /IANS