BRITAIN on Friday(27) asked its regulator to assess AstraZeneca's Covid-19 vaccine for a rollout after experts raised questions about trial data.
Meanwhile, the company said that it may run another study to gauge the shot's efficacy.
The UK government has secured 100 million doses of the vaccine, developed by AstraZeneca and Oxford University.
The British drugmaker expects 4m doses to be available in the country by the end of next month, and health secretary Matt Hancock aims for a rollout to begin before Christmas.
"We have formally asked the regulator to assess the Oxford/AstraZeneca vaccine, to understand the data and determine whether it meets rigorous safety standards," Hancock said.
"This letter is an important step towards deploying a vaccine as quickly as safely possible."
Britain's Medicines and Healthcare products Regulatory Agency (MHRA) started an accelerated "rolling review" of the vaccine at the start of this month as data comes in on safety and efficacy.
AstraZeneca had released trial data on Monday(23) that showed its experimental vaccine prevented on average 70 per cent of Covid-19 cases in late-stage trials in Britain and Brazil.
While the success rate was 90 per cent in the sub-group, some experts said the relatively small number of participants made it harder to be confident in the findings.
Only 2,741 volunteers were in the sub-group of the AstraZeneca-Oxford trial that gave the 90 per cent efficacy read-out, a fraction of the tens of thousands in trials that resulted in the above 90 per cent efficacy data released earlier this month for Pfizer-BioNTech's and Moderna's vaccines.
AstraZeneca said the administering of the half dose in the trial had been reviewed and approved by independent data safety monitors and the UK regulator, adding that the regulator publicly confirmed there was "no concern".
CEO Pascal Soriot said on Thursday(26), though, that the drugmaker was likely to run an additional global trial to assess the efficacy of its vaccine using the lower dosage.
Pauline Londeix, co-founder of French drug transparency group OT-Med, said the apparent confusion over the trial results was "very problematic for public confidence in vaccines".
Meanwhile, Britain's top science adviser said on Thursday that the interim results showed the AstraZeneca vaccine was successful.
AstraZeneca's vaccine is viewed as the best hope for many developing countries because of its cheaper price and ability to be transported at normal fridge temperatures.
Officials in the Philippines said on Friday they would secure 2.6m shots of the AstraZeneca shot - the country's first supply deal for a Covid-19 vaccine - and were negotiating a possible purchase of a further 1m doses.
The announcements came despite some scientists raising doubts about the robustness of results showing the shot was 90 per cent effective in a sub-group of trial participants who, by error initially, received a half dose followed by a full dose.
AstraZeneca's stock price drop this week, Moderna has rallied 22 per cent since releasing its vaccine trial data on Nov. 16 and Pfizer and BioNTech are up 6 per cent and 14 per cent respectively since announcing their successful data on Nov. 9.
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