There was more apparent good news this week on the race to develop a vaccine against the coronavirus.

The drug company AstraZeneca said in a press release that its AZD1222 vaccine candidate had proved to be “highly effective” in preventing Covid-19, based on early results from clinical trials in the UK and Brazil.

The vaccine is being developed in collaboration with scientists at Oxford University, with financial backing from the UK government.

The results were widely reported as good news. But the headline figures for how many infections the vaccine prevented were slightly lower and harder to understand than other vaccines that have recently reported trial results.

Now the Oxford vaccine has been criticised by some in the scientific community for offering incomplete and confusing data, and only belatedly admitting to an error that changed the results.

Some scientists have even said that they doubt whether the British vaccine would be approved by regulators in the US.

Let’s try to put all this in context.

Why is the Oxford/AstraZeneca vaccine under fire?

AstraZeneca put out a press release saying its vaccine prevented up to 90 per cent of Covid-19 infections, depending on how much vaccine people were given.

This was a less clear-cut result than those offered by Pfizer and Moderna, who are also developing vaccines and claim 95 per cent efficacy – a calculation based on new coronavirus infections among people vaccinated, compared to those given a placebo.

In the Oxford clinical trials, two full doses given a month apart prevented 62 per cent of cases. When they halved the first dose, efficacy rose to 90 per cent. Averaging the two gave an overall efficacy of 70 per cent.

While the team are looking at data from more than 23,000 participants in total, only 2,741 people got the dose that protected against 90 per cent of infections.

Some scientists, like former senior Pfizer executive John LaMattina, have said they doubt whether US regulators would grant emergency use authorization to AstraZeneca on the bases of such a small number, saying: “More data for this dosing (regimen) will be needed.”

Dosing error

There’s a second issue: AstraZeneca only admitted after it had published the initial press release that the difference in dosage given to different groups of trial participants stemmed from an error in the manufacturing process.

Mene Pangalos, head of drug research at AstraZeneca, played down the mistake, calling it “serendipity.”

A spokesman for the Oxford researchers said the team spotted the error and raised it with regulators.

They agreed that the scientists could test both the lower and higher dose groups and include the results on the phase III trial.

The researchers say the dosing problem has been fixed now, but they say they don’t yet know why giving people a lower first dose proved, apparently, to be more effective.

What do independent experts say?

Dr Joy Leahy, Royal Statistical Society Statistical Ambassador, told the Science Media Centre: “The dosing error does throw up some problems.

“Taking any trial, the more different dosing regimens that are in the trial, the more likely it is that any one regimen will turn out to have a particularly high efficacy, just because of chance and random variation.”

The scientist Hilda Bastian has been particularly critical of AstraZeneca’s approach, writing in Wired that its results come from “patched-together analysis instead of data from a single, large trial” like Pfizer and Moderna.

Others have defended the Oxford team’s approach, pointing out that regulators approved the decision to include the lower dose group in the trial and that efficacy in the high dose group was still above 50 per cent – often considered the bar for a successful vaccine.

Most scientists say they are suspending judgement until the full peer-reviewed results are published.

With only the scant details of a short press release to go on, there are many unanswered questions about the data.

Why don’t we have the full data yet?

There’s long been criticism of the way drug companies announce selective results from trials in press releases, instead of publishing the full data in peer-reviewed journals.

The incomplete information often leaves scientists scratching their heads over how to interpret the data. Issuing press releases ahead of a proper data release is, however, standard practice.

Professor Stephen Evans, Professor of Pharmacoepidemiology, London School of Hygiene & Tropical Medicine, said in a statement: “When the trial data indicate that the vaccine is likely to meet the criteria for authorization of use then they have to inform the investigators and the companies.

“At that stage because there could be leaks of confidential information which might result in insider trading, the sponsors and investigators have to put out a press release with minimal information because of stock market law.

“This of course is not ideal from a scientific point of view but has become necessary in this pandemic.”

He added: “The way the data are put together will have been specified in the protocol and scrutinised very carefully by regulators to ensure that there is no “cherry picking” of the results.

“We have good grounds for trusting that the regulation in this high-profile area will be done as carefully or more carefully for these vaccines than for any others in the past.”