In authorizing the vaccine, United Kingdom regulator accepted the pooled results, but didn't approve administering the half-dose/full-dose regimen.
The data from the study by the University of Oxford, which has not yet been peer reviewed, supports the four to 12-week prime-boost dosing interval that many global regulators, including the UK's, have recommended.
A single standard dose of the vaccine is 76% effective at protecting from primary symptomatic COVID-19 for the first 90 days post vaccination, once the immune system has built this protection 22 days after the vaccination, with the protection showing little evidence of waning in this period.
The paper said that vaccine efficacy was 82.4% with 12 or more weeks to the second dose, compared to 54.9% for those where the booster was given under 6 weeks after the first dose.
Shenzhen Kangtai Biological Products said on Tuesday it had completed a facility created to be able to produce 400 million doses of AstraZeneca's COVID-19 vaccine per year, doubling a capacity target promised in 2020.
Egypt's Health and Population Ministry said that the 50,000 doses received on Sunday are sufficient for 25,000 people. The researchers also found that neutralising antibody titres measured by pseudovirus were higher after a longer interval before the second dose.
The Russian vaccine, which works in the same way as the Oxford jab, was found to be 74 per cent effective at blocking Covid after just a single dose and worked for all ages.
British Health Secretary Matt Hancock said: "This is a hugely encouraging study and further reinforces our confidence that vaccines are capable of reducing transmission and protecting people from this very bad disease". Some of the participants who initially consented to a single dose study did not choose to receive the second dose, providing a self-selected cohort of single-dose recipients. "Together with the new findings on reduced transmission, we believe this vaccine will have a real impact on the pandemic". "In addition, data from an additional month of follow-up is now available for inclusion in the analysis, providing greater precision in estimates due to the larger number of cases for analysis in comparison with the previous report", they write.
The letter, signed by chief investigator Pollard, stated that researchers "are not sure what dose of vaccine is most likely to be protective against COVID disease" and explained that doses "are measured using standard scientific test methods". The data covers the period up to December 7, 2020.
'But the outcome reported here is clear and the scientific principle of vaccination is demonstrated, which means another vaccine can now join the fight to reduce the incidence of Covid-19'.
Russian Federation became the first country to register a Covid vaccine for emergency use back in August, despite it only having been tested on a handful of people.