INTERIM RESULTS OF the Oxford Covid-19 vaccine’s Phase 3 trials have found it protects against symptomatic disease in 70% of cases, with efficacy of 90% in a smaller subset of participants who were given a lower dose.

Just three out of 23,745 participants over a median of 3.4 months experienced serious adverse events that were possibly related to a vaccine. All three participants have recovered or are recovering and remain in the trial.

The results, published in The Lancet, are the first full peer-reviewed efficacy results to be published for a Covid-19 vaccine.

They are based on analysis of seven trials in the UK, Brazil and South Africa. Half the participants were given the Covid-19 vaccine and the other half were given a control (either a meningococcal conjugate vaccine or saline). 

Out of 23,745 participants, 168 experienced a total of 175 severe adverse events over the period but 172 of those events were unrelated to either the Covid-19 or control vaccines. Just one of the three serious adverse events was in the Covid-19 vaccine group – a case of transverse myelitis [ inflammation of a section of the spinal cord]. 

All three participants have recovered or are recovering, the study author has said, and they are all still part of the trial.

There were 131 cases of symptomatic Covid-19 disease more than 14 days after the second vaccine dose among 11,6356 participants.

 The trial was originally designed to assess a single dose, but following review of the immune response data in earlier phases, another dose was added to the trial protocol. Most participants received two doses, each a standard dose.

However a subset (1,367 people) in the UK received a half dose as their first dose and then a full second dose. This was because of differences in the results of quantification methods between batches of the vaccine.

Authors found that their analyses of different doses suggests higher efficacy among those who had a half dose and then a standard dose. They have said this requires further research as more data becomes available from the trial.

Those who received two full doses saw an efficacy of 62%, while those who received a half dose and then a full second dose saw an efficacy of 90%.

In the 11,636 people included in the vaccine efficacy against symptomatic disease analysis, 12% (1,418/11,636 people) were older adults and most were white (83%, 9,625/11,636 people).

Most participants were aged 18-55 as those aged 56 years and older were recruited later and will be studied in future analyses. 

“In order to assess vaccine efficacy, we need to have a sufficient number of Covid-19 cases among participants to indicate that the vaccine is protecting them from disease. Since recruitment of older adults started later than in younger adults there has been less follow up time for these cohorts and less time to accrue Covid-19 cases,” study author, Dr Merryn Voysey, University of Oxford said.

“This means we have to wait longer to have sufficient data to provide good vaccine efficacy estimates in smaller subgroups.”

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 and all of these were in control groups – those who did not receive the Covid-19 vaccine.

 The trial also measured protection against asymptomatic infection by asking 6,638 UK participants to complete weekly Covid tests. There were 69 cases of asymptomatic Covid-19 disease identified in the UK study’s weekly Covid-19 testing of 6,638 people, leading to a vaccine efficacy against asymptomatic infection of 27%.

Authors noted they are not yet able to assess the duration of protection as first trials were initiated in April 2020 and all disease episodes have accrued within six months of the first dose being administered.