In a significant development, the first full results from interim analysis has confirmed that the Oxford COVID-19 vaccine has an acceptable safety profile and is efficacious against symptomatic the Coronavirus caused disease, with only three out of 23,745 participants experiencing serious adverse events.

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Interim results of the Oxford COVID-19 vaccine trials found that the vaccine protects against symptomatic disease in 70 percent of cases -- with vaccine efficacy of 62 percent for those given two full doses, and of 90 percent in those given a half then a full dose. The results are the first full peer-reviewed efficacy results to be published for a COVID-19 vaccine, and are published in The Lancet. The results were based on a pre-specified pooled analysis of phase 3 trials in UK and Brazil (11,636 people), alongside safety data from a total of 23,745 participants in 4 trials in the UK, Brazil and South Africa.

All participants have recovered or are recovering, and remain in the trial. "The results presented in this report provide the key findings from our first interim analysis," said study author and doctor Merryn Voysey from University of Oxford. "In future analyses, with more data included as it becomes available, we will investigate differences in key subgroups such as older adults, various ethnicities, doses, timing of booster vaccines, and we will determine which immune responses equate to protection from infection or disease," the author said.

The Oxford COVID-19 vaccine uses a chimpanzee adenovirus viral vector that cannot cause disease in humans and expresses the SARS-CoV-2 spike protein. This means the vaccine delivers the spike protein genetic code into vaccinated people's cells, which then produce the protein, and teaching the immune system to recognise and attack the virus. Past trial results have found that the vaccine induces antibody and T cell immune responses, and is safe in adults aged 18 years and over, including older adults, the study noted. "Our findings indicate that our vaccine's efficacy exceeds the thresholds set by health authorities and may have a potential public health impact," said study lead author Professor Andrew Pollard.

Cases of severe disease and hospitalisation were monitored for in all 23,745 participants. From 21 days after the first dose there were 10 cases hospitalised for COVID-19, all in the control arm, and two were classified as severe, including one death. "We have shown for the first time that an adenoviral vectored vaccine - a type of vaccine technology which has been in use since 2009 - is efficacious and could contribute to disease control in the COVID-19 pandemic," said Professor Sarah Gilbert from University of Oxford.

The authors noted that they are not yet able to assess duration of protection, as the first trials were initiated in April 2020 and all disease episodes have accrued within six months of the first dose being administered. Further evidence will be required to determine duration of protection and the need for additional booster doses of vaccine, the authors said.