Trials of a Covid-19 vaccine being developed by AstraZeneca and Oxford University will resume after being paused due to a reported side effect in a patient in the UK.
On Tuesday, AstraZeneca said the studies were being paused while it investigated whether the adverse reaction was linked with the vaccine.
But on Saturday, the university said it had been deemed safe to continue.
The patient involved in the study had been reportedly suffering from neurological symptoms associated with a rare spinal inflammatory disorder called transverse myelitis.
AstraZeneca, based in Cambridge, said it could not disclose further medical information.
Health Secretary Matt Hancock welcomed the news that the trials would resume.
"This pause shows we will always put safety first. We will back our scientists to deliver an effective vaccine as soon as safely possible," he added.
The university said in a statement that it was "expected" that "some participants will become unwell" in large trials such as this one.
It added that the studies could now resume following the recommendations of an independent safety review committee and the UK regulator, the Medicines and Healthcare Products Regulatory Agency (MHRA).
The World Health Organization (WHO) says nearly 180 vaccine candidates are being tested around the world but none has yet completed clinical trials.
Hopes have been high that the vaccine might be one of the first to come on the market, following successful phase 1 and 2 testing.
Its move to Phase 3 testing in recent weeks has involved some 30,000 participants in the US as well as in the UK, Brazil and South Africa. Phase 3 trials in vaccines often involve thousands of participants and can last several years.
The government's chief scientific adviser, Sir Patrick Vallance, told the Downing Street press conference on Wednesday what had happened in the Oxford trial was not unusual.
The Serum Institute of India said it would restart its trials once it had permission from the Drugs Controller General of India.
Brazil's health regulator ANVISA said it was awaiting notice from the British MHRA confirming that resumption of trials has been authorized before resuming in Brazil.
The Federal University of Sao Paulo, which is conducting the paused trials, said in a statement that 4600 of the planned 5000 volunteers have been recruited and vaccinated without any of them reporting any serious health issues.
Governments around the world are desperate for a vaccine to help end the pandemic, which has caused more than 900,000 deaths and global economic turmoil. The WHO had flagged AstraZeneca's as the most promising.
The vaccine is in late-stage clinical trials in the United States, Britain, Brazil and South Africa and additional trials are planned in Japan and Russia.
The pause of the trials came after reports that the United States was aiming for fast-track authorization or approval of a vaccine before November's presidential election.
Leading US and European vaccine developers have pledged to uphold scientific safety and efficacy standards for their experimental vaccines and not bow to political pressures to rush the process.
AstraZeneca has already agreed to supply close to 3 billion doses to governments across the globe - more than any other vaccine project.
The WHO's chief scientist said the pause in the trials should serve as a "wake-up" call that there would be ups and downs in the development of a vaccine.
"Inevitably with such a large trial there will be times when safety issues arise," said Peter Openshaw, professor of experimental medicine at Imperial College London.
"We must all hope that there are no future events and that the vaccine proves both safe and effective."
- BBC / Reuters