AstraZeneca PLC (LON:AZN) is facing criticism over the results from its COVID-19 vaccine candidate trials, with some questioning whether it will receive US approval.

The jab showed to have an average efficacy of 70% in late-stage studies conducted in the UK and Brazil.

READ: Will AstraZeneca rebound after slipping on Monday’s COVID-19 vaccine news?

That’s because when volunteers were given a half dose followed by a full dose at least one month apart, the chance of being protected from the virus was 90%, which fell to 62% when administered as two full doses at least one month apart.

However, a US government official revealed that in the former trial all participants were younger than 55, meaning they were less at risk from experiencing severe symptoms, though the FTSE 100 group did not disclose this fact.

It is not clear why the lower dose, which is understood to have been administered by mistake, provokes a higher immune response.

Sarah Gilbert, one of the Oxford professors leading the project, said that the smaller dose may kickstart the body into developing an immune memory.

“There are a number of variables that we need to understand,” Moncef Slaoui, the head of the White House’s Operation Warp Speed, was reported as saying by The Times.

“The 90% efficacy group and the 62% efficacy group are overlapping statistically, so it is still possible that that difference is a random difference. It’s unlikely but it’s still possible it’s a random difference.”

John LaMattina, former senior executive at COVID-19 vaccine competitor Pfizer Inc. (NYSE:PFE), said it is “hard to believe” that the US authorities will issue an emergency use approval since the jab’s optimal dose was only given to 2,300 people.

Mene Pangalos, AstraZeneca’s executive vice president for research, said the mistake is “irrelevant”, the Daily Mail reported.

“Whichever way you cut the data – even if you only believe the full-dose, full-dose data… We still have efficacy that meets the thresholds for approval with a vaccine that’s over 60% effective,” he said.

The World Health Organisation and the US Food and Drug Administration are requiring an efficacy rate of 50% for COVID-19 inoculations.

Shares were flat at 7,791p on Thursday afternoon.

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