LONDON (AP) — The British government said Friday it has formally asked the country’s medicines regulator to assess whether a coronavirus vaccine developed by AstraZeneca and Oxford University should be authorized for use.

The step comes amid questions about preliminary results from trials of the jab, after the company and the university acknowledged that the most encouraging part of their findings stemmed from a dosing error.

U.K. Health Secretary Matt Hancock said he had asked the Medicines and Healthcare Products Regulatory Agency to determine whether the vaccine “meets rigorous safety standards.”

It’s the second vaccine candidate to reach the formal assessment stage in Britain, following a shot developed by Pfizer and its German partner BioNTech. A third vaccine from U.S. firm Moderna is not far behind.

The British government has ordered 100 million doses of the Oxford-AstraZeneca vaccine, and plans to start distributing it in December if it gains approval.

The regulator said it could not give a time frame for possible approval of the vaccines

MHRA Chief Executive June Raine said “no vaccine would be authorized for supply in the U.K. unless the expected standards of safety, quality and efficacy are met.”

Oxford and AstraZeneca reported Monday that their vaccine appeared to be 62% effective in people who received two doses, and 90% effective when volunteers were given a half dose followed by a full dose. They did not mention at the time, but later acknowledged, that a manufacturing issue had resulted in “a half dose of the vaccine being administered as the first dose” to some participants.

The drugmakers informed the U.K. regulator of the issue when it was discovered, and it was agreed to complete the late-stage trial with two groups.

AstraZeneca has said it plans to conduct a new global clinical trial to assess the vaccine’s efficacy but does not expect that to delay regulatory approval in Britain or the European Union — though the U.S. Food and Drug Administration may take longer.

Some scientists have expressed concerns about gaps in the data and the way the results were reported. Only 2,741 people received the half dose, making it hard to know if the effectiveness seen in the group is real or a statistical quirk. A total of 8,895 people received two full doses.

Eleanor Riley, professor of Immunology and Infectious Disease at the University of Edinburgh, said Oxford and AstraZeneca needed to answer questions about their results “clearly and completely.”

“Trust is at a premium when it comes to vaccines and we must not do anything that might in any way undermine that trust,” she said.

Full results are due to be published in medical journal The Lancet, though no date has been given.

Pfizer and BioNTech said earlier this month that their vaccine is 95% effective, and Moderna said its product appears to be 94.5% effective, according to preliminary data.

Unlike the Pfizer and Moderna vaccines, the Oxford-AstraZeneca jab does not need to be stored at freezer temperatures, making it potentially easier to distribute, especially in developing countries. It is also cheaper, because AstraZeneca has agreed not to profit from it during the pandemic.

The British government and its scientific advisers have expressed confidence that multiple vaccines will win approval, but say all decisions are up to the regulator.

“They will make an assessment with lots of data that is not currently public domain on efficacy and on safety,” said England’s Chief Medical Officer, Chris Whitty.

“I think it’s always a mistake to make too many judgements early before we have the full information and particularly before the regulator, the independent regulator, has had their chance to look at the data and make an assessment.”