The Oxford study also found that one vaccine shot provides 76 percent effective protection against symptomatic infection, supporting the UK government’s decision to allow a longer 12-week gap between two shots.
The results showed that the number of vaccine doses administered in the UK exceeded 10 million, with 9,646,715 first doses and 496,796-second doses being transferred to healthcare workers and vulnerable groups.
The vaccine is also being shipped worldwide, with the first doses arriving in Dubai on Tuesday.
This is the first study to show that the Covid-19 vaccine can reduce transmission of the virus.
Professor Andrew Pollard, Principal Investigator for the Oxford Vaccine Group, said the results indicated that the Oxford vaccine should have a “huge impact on transmission” in the UK.
“The number of people vaccinated who are PCR [polymerase chain reaction] positive and therefore infected has dropped by about two-thirds,” he told BBC Radio 4 Today.
“Since they are no longer infected, they cannot transmit the virus to other people. This should have a huge impact on transmission. “
However, Professor Pollard warned that the study did not consider new variants of the virus, such as mutations first identified in South Africa and Brazil.
“This virus is absolutely trying to find ways to continue transmission despite human immunity,” he said.
The new mutation, found in parts of the UK, is causing concern for scientists.
Two shots “sweet spot”
The study measured the impact on the transmission of the virus by testing for asymptomatic infections, taking swabs from participants every week, and registering cases of Covid-19.
The results, compiled from trials in the UK, Brazil, and South Africa, also showed that immune responses increased at longer intervals for the second dose in participants between 18 and 55.
Study leader AstraZeneca said 8-12 weeks between doses appears to be a “golden spot” for efficacy, unlike US drug manufacturer Pfizer, which warned that the vaccine, developed with Germany’s BioNTech not tested during this interval.
The new study did not address concerns about a lack of efficacy data among older adults, who the British government prioritizes in vaccine introduction.
Professor Pollard said the data showed that a 12-week interval between doses was “the best approach to the deployment and assures us that people are protected 22 days after a single dose.”
According to Oxford, the results of the pre-printed article, which has not been peer-reviewed, support the UK’s decision to extend the interval between initial and booster doses to 12 weeks.
“The vaccine efficacy after a single dose of the vaccine between 22 and 90 days after vaccination was 76 percent, and the simulation showed that protection did not weaken during this initial three-month period,” the Oxford scientists said in a preprint.
The document says the vaccine was 82.4% effective 12 weeks or more before the second dose, compared with 54.9% for those boosted less than six weeks after the first dose.
The longest dose interval for individuals aged 56 years and older was six to eight weeks, so there was no evidence of the effectiveness of a 12-week dosage break in this cohort.
The UK has expressed confidence that the vaccine works in all age groups.
The study found that none of the 12,408 people vaccinated with a single dose of the vaccine were hospitalized with Covid-19 22 days after immunization.