In large trials like this one, "it is expected that some participants will become unwell and every case must be carefully evaluated to ensure careful assessment of safety," the university said.
ADVERTISEMENTOxford University on Saturday said trials of a COVID-19 vaccine it is developing with pharmaceutical company AstraZeneca will resume.
It came after AstraZeneca on Tuesday said the trials were being put on hold while it investigated whether a reported side effect in a UK patient was connected with the vaccine.
In large trials like this one, "it is expected that some participants will become unwell and every case must be carefully evaluated to ensure careful assessment of safety," the university said.
Around 18,000 individuals have received study vaccines globally as part of the trial, it added.
The university has not disclosed medical information about the illness that caused the trial to be put on hold, citing participant confidentiality.
The institution said it is "committed to the safety of our participants and the highest standards of conduct in our studies and will continue to monitor safety closely."
Health experts, including the government's chief scientific adviser, Sir Patrick Vallance, have said pauses in drug trials are commonplace to ensure safety and effectiveness.
Oxford University said that the studies could now start again after an independent safety review committee and the UK's Medicines and Healthcare Products Regulatory Agency had given their recommendations.
Health Secretary Matt Hancock said on Twitter: "This pause shows we will always put safety first. We will back our scientists to deliver an effective vaccine as soon as safely possible."
After completing Phase 1 and 2 testing successfully, many are hopeful that that vaccine will be the first one to hit the market.
The Phase 3 stage of vaccine trials can involve thousands of subjects, often spanning several years.