The data, which have yet to be peer reviewed, also found that a single dose of the jab is 76% effective at preventing symptomatic COVID-19 from 22 days after being administered and that this level of protection continues until 90 days post-vaccination. After a second dose is administered at three months efficacy increased to 82.4%.
This overall efficacy figure is higher than from earlier trial results when the jab was administered with a four-week gap, which found it was 62% effective at preventing symptomatic infection. The results appear to back the UK government's decision to move to a longer gap between doses of this particular vaccine in a bid to give more people a first jab.
The latest results of the trial are presented in a pre-print paper in the Lancet. It covers phase 3 data from the UK and Brazil and phase 1 and 2 data from the UK and South Africa in 17,1777 participants up to 7 December 2020.
Longer gap between doses boosts efficacy
The paper found that a longer gap between doses increased the vaccine's effectiveness. It suggests that vaccine efficacy is around 55% when a second dose is given no more than six weeks after the first - but this increases to 82.4% when the second dose is administered more than three months later.
The study also looked at transmission by undertaking PCR swab tests every week in UK participants to test for asymptomatic infection as well as monitoring those people who fell ill.
The researchers said that the data 'show that a single standard dose of the vaccine reduced PCR positivity by 67%, and that, after the second dose, the [single dose/single dose] schedule reduced PCR positivity by 49.5% overall'. The overall reduction in positive PCR tests was 54.1%.
The data indicate that the vaccine 'may have a substantial impact on transmission by reducing the number of infected individuals in the population', the researchers added.
The study found there were no hospitalisations in any person who received the vaccine 21 days after the first jab was administered – there were 15 in the control group. It also found that antibody levels remained relatively constant between 21 and 90 days post vaccination.
The researchers concluded that a vaccination programme with the Oxford/AstraZeneca vaccine 'aimed at vaccinating a large proportion of the population with a single dose, with a second dose given after a three-month period is an effective strategy for reducing disease, and may be the optimal for rollout of a pandemic vaccine when supplies are limited in the short term'.
Professor Andrew Pollard, chief investigator of the Oxford Vaccine Trial, and co-author of the paper said: ‘These new data provide an important verification of the interim data that was used by more than 25 regulators including the MHRA and EMA to grant the vaccine emergency use authorisation.
‘It also supports the policy recommendation made by the Joint Committee on Vaccination and Immunisation (JCVI) for a 12-week prime-boost interval, as they look for the optimal approach to roll out, and reassures us that people are protected from 22 days after a single dose of the vaccine.’
On Twitter health and social care secretary Matt Hancock said results from the study were 'absolutely superb'. He later added: 'Vaccines are the way out of this pandemic and we are making fantastic progress vaccinating the most vulnerable.'
This news about the Oxford vaccine is absolutely superb.— Matt Hancock (@MattHancock) February 2, 2021
?? 2/3 reduction in transmission
?? Stronger protection from 12 week gap between doses
?? No hospitalisations
This vaccine works & works well ????