Two doses of the Oxford-AstraZeneca COVID-19 vaccine may be around 85% to 90% effective against symptomatic disease, Public Health England (PHE) said on Thursday.

PHE said the preliminary findings were the first of its kind on the effectiveness of two doses of AstraZeneca in a real-world setting but cautioned that it had "low confidence" in the findings, and the results would be inconclusive until more evidence was gathered.

In a weekly surveillance report, Public Health England said the estimated effectiveness of the AstraZeneca vaccine, invented at the University of Oxford, was 89% compared to unvaccinated people.

That compares to 90% estimated effectiveness against symptomatic disease for the Pfizer/BioNTech vaccine.

The AstraZeneca vaccine is also manufactured by the Serum Institute of India in Pune under the Covishield brand and is the mainstay of India’s inoculation drive as around 70 million doses are produced every month. The country’s homegrown vaccine, Bharat Biotech’s Covaxin, is the second vaccine of which 10 million doses are being currently produced at Hyderabad. This will now be stepped sharply as the company will start producing the vaccine at it its Bengaluru and Gujarat plants as well. Besides, the four public sector companies will also start rolling out the doses.

Britain has had one of the fastest vaccine rollouts, generating a lot of data about the use of the shots in real-world settings.

It was the first country to roll out AstraZeneca's vaccine, which faced questions over the construction of its clinical trials, the efficacy of the vaccine and the optimal gap between doses of its shot.

AstraZeneca welcomed the preliminary findings. "This latest real-world data from PHE adds to the growing body of evidence that demonstrates the effectiveness of our vaccine against COVID-19," a spokesman said.

Britain has also been using the shots manufactured by Pfizer and AstraZeneca since December and January respectively, and in April also started administering shots of the Moderna vaccine.

PHE said there was a "small reduction in vaccine effectiveness" from 10 weeks after the first dose of the Pfizer shot before the second shot is given.

Britain extended the gap between doses to 12 weeks, though Pfizer warned there was a lack of evidence of its efficacy outside the three-week gap used in trials.

Last week, Britain cut the gap between doses of Pfizer vaccines down to 8 weeks for the over 50s because of concern over the B.1.617.2 variant first found in India.