Oxford/AstraZeneca Covid vaccine ‘dose error’ explained


On Monday, the world heard how the UK’s Covid vaccine – from AstraZeneca and Oxford University – was highly effective in advanced trials.

It gave hope of another new jab to fight the pandemic that should be cheaper and easier to distribute than the Pfizer/BioNTech and Moderna mRNA vaccines that announced similarly impressive results just days before.

But after the jubilation, some negative press has followed.

On Thursday, multiple news outlets in the UK and US reported that there were questions over the data. They weren’t about safety, but rather how effective the jab is.

The questions centre around efficacy levels.

Three were reported from the trial – an overall efficacy of 70%, a lower one of 62% and a high of 90%.

That’s because different doses of the vaccine were mistakenly used in the trial. Some volunteers were given shots half the planned strength, in error. Yet that “wrong” dose turned out to be a winner.

What does that mean?

Some of the shots were weaker than they were designed to be, containing much less of the ingredient that is meant to give a person immunity.

The jab is actually two shots, with the second given a month after the first as a booster.

While most of the volunteers in the trial got the correct dose for both of their two shots, some didn’t.

Regulators were told about the error early on and they agreed that the trial could continue and more volunteers could be immunised.

The error had no effect on vaccine safety.

What were the results?

About 3,000 participants were given the half dose and then a full dose four weeks later, and this regime appeared to provide the most protection or efficacy in the trial – around 90%.

In the larger group of nearly 9,000 volunteers, who were given two full doses also four weeks apart, efficacy was 62%.

AstraZeneca reported these percentages and also said that its vaccine was, on average, 70% effective at preventing Covid-19 illness. The figures left some experts scratching their head.

Prof David Salisbury, immunisation expert and associate fellow of the global health program at the Chatham House think tank, said: “You’ve taken two studies for which different doses were used and come up with a composite that doesn’t represent either of the doses. I think many people are having trouble with that.″

AstraZeneca stressed that the data are preliminary, rather than full and final – which is true for the reported Pfizer and Moderna jab results too. It is science by press release.

When they can, all of the companies will publish full results in medical journals for public scrutiny.

And they are submitting full data to regulators to apply for emergency approval so that countries can start using these three different vaccines to immunise whole populations.


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