Britain gave AstraZeneca's Covid-19 vaccine a vote of confidence on Friday when it asked its regulator to assess it for a rollout after experts raised questions about trial data and the company said 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 most supplies it has ordered of any shot to fight the pandemic.
The British drugmaker expects 4 million 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.
In the global race to develop vaccines against Covid-19, AstraZeneca's candidate is viewed as offering one of the best hopes 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 2.6 million 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 1 million doses.
The announcements came despite some scientists raising doubts about the robustness of results showing the shot was 90% effective in a sub-group of trial participants who, by error initially, received a half dose followed by a full dose.
AstraZeneca had released trial data on Monday that showed its experimental vaccine prevented on average 70% of Covid-19 cases in late-stage trials in Britain and Brazil.
While the success rate was 90% in the sub-group, some experts said the relatively small number of participants made it harder to be confident in the findings.
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, 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".
"It has largely to do with the race drugmakers are engaged in currently, which leads them to present vaccine candidates in the best possible way and not release full protocols and results. It is the opposite of what is needed in our view."
Nonetheless, Britain's top science adviser said on Thursday that the interim results showed the AstraZeneca vaccine was successful.
"The headline result is the vaccine works and that's very exciting," Patrick Vallance said during a news conference with Prime Minister Boris Johnson.
Shares in AstraZeneca were down 0.8% at around 0830 GMT. They have fallen about 7% since it reported the vaccine data on Monday.
Only 2,741 volunteers were in the sub-group of the AstraZeneca-Oxford trial that gave the 90% efficacy read-out, a fraction of the tens of thousands in trials that resulted in the above 90% efficacy data released earlier this month for Pfizer-BioNTech's and Moderna's vaccines.
"Sub-group analyses in randomised controlled trials are always fraught with difficulties," said Paul Hunter, a professor of medicine at the Britain's University of East Anglia.
"In order to have faith in the results," Hunter said, any sub-group analysis "should be sufficiently powered" with large numbers of volunteers to take readings from.
'NUMBER OF VARIABLES'
In contrast to AstraZeneca's stock price drop this week, Moderna has rallied 22% since releasing its vaccine trial data on Nov. 16 and Pfizer and BioNTech are up 6% and 14% respectively since announcing their successful data on Nov. 9.
The U.S. regulator, the Food and Drug Administration (FDA), has not commented on AstraZeneca's vaccine trial results. The European Medicines Agency said on Thursday it would "assess data on the efficacy and safety of the vaccine in the coming weeks once they have been received from the company".
Moncef Slaoui, chief scientific adviser for the U.S. government's vaccine programme Operation Warp Speed, has also highlighted gaps in the trial data.
He said no-one in the subgroup that got the initial half dose was older than 55 - suggesting the regimen's efficacy in crucial older age groups is unproven in this interim data.
In the group that received a correct full dose followed by a full dose, he noted, older people were included.
"There are a number of variables that we need to understand, and what has been the role of each one of them in achieving the difference in efficacy," Slaoui told a briefing on Tuesday.