Dosing error in trials led to Oxford vaccine's 90 per cent efficacy by accident, say scientists

Sarah Knapton
·4-min read
Professor Katie Ewer, senior immunologist for Oxford's Ebola and pre-erythrocytic malaria vaccine trials. Professor Ewer is part of the team working on the coronavirus vaccine developed by AstraZeneca and Oxford University - John Cairns/John Cairns
Professor Katie Ewer, senior immunologist for Oxford's Ebola and pre-erythrocytic malaria vaccine trials. Professor Ewer is part of the team working on the coronavirus vaccine developed by AstraZeneca and Oxford University - John Cairns/John Cairns
Coronavirus Article Bar with counter
Coronavirus Article Bar with counter

A dosing error during clinical trials led to the Oxford vaccine reaching 90 per cent efficacy by accident, it has emerged. 

In the spring, scientists were left baffled as to why participants were experiencing much milder side effects than expected. 

When they checked, they found participants had received just half the dose given to 500 adults in earlier safety trials. 

Instead of restarting the trial, researchers at Oxford University boosted the initial participants with a full dose while everyone who enrolled later received the full amount.  

Trial results announced on Monday show that the 'correct' vaccine dose achieved just 62 per cent efficacy while the 'accidental' arm was 90 per cent effective.

Sir Menelas Nicolas Pangalos, executive vice president of biopharmaceuticals research and development at AstraZeneca, the vaccine manufacturer, said: “The reason we had the half-dose is serendipity.

“We went back and checked ... and we found out that they had underpredicted the dose of the vaccine by half.”

Once informed, regulators allowed the trial of more than 20,000 volunteers to continue – leading to results which were on Monday evening hailed by Prime Minister Boris Johnson as "incredibly exciting news".

Deputy chief medical officer for England, Professor Jonathan Van-Tam said he shared “the euphoria” of scientists over a third vaccine - Pippa Fowles/Reuters
Deputy chief medical officer for England, Professor Jonathan Van-Tam said he shared “the euphoria” of scientists over a third vaccine - Pippa Fowles/Reuters

Deputy chief medical officer for England, Professor Jonathan Van Tam, on Monday said he shared “the euphoria” of scientists now that a third major vaccine had been shown to work.

AstraZeneca said 20 million doses could be available by the start of next year – with four million already waiting for use.

The findings about the efficacy of the half-doses mean there could be enough for eight million people to begin their vaccination programme before Christmas. 

In addition, up to five million doses of vaccines by Pfizer could be administered next month.

Unlike the Pfizer jab, the vaccine from AstraZeneca can be kept at normal fridge temperatures, making it easier to store and distribute.  

Ministers hope both jabs could begin being rolled out next month, with the NHS on standby to be ready to start administering vaccines from December 1.

The Medicines and Healthcare products Regulatory Agency (MHRA) is expected to be asked to begin assessing the AstraZeneca vaccine within days. 

On Monday, developers Pfizer/BioNTech handed over full safety and efficacy data to the MHRA allowing assessment of its jabs to get underway. 

In total, Britain has ordered 40 million doses of the Pfizer jab, and 100 million of the AstraZeneca vaccine. In both cases, two doses have to be given, a month apart.

However, the findings about the AstraZeneca vaccine may mean its doses can stretch even further – with only a half dose required for the first jab. 

Tom Keith-Roach, the president of AstraZeneca UK, said 19 million doses could be ready by the end of the year, including the four million doses that are already on standby.

He said the slew of recent trial results were a major step forward in the battle to end the global pandemic.

He said: "This is clearly not the end of the war, but it's a bit like the discovery of radar in the Second World War to me.  

“Now we actually have the technology that we need to fight this virus and win.”

On Monday, Health Secretary Matt Hancock said: "If this all goes well in the next couple of weeks, then we are looking at the potential of starting the vaccination programme next month for this Oxford-AstraZeneca vaccine as well as the Pfizer vaccine. But in all cases the bulk of the rollout will be in the New Year.”

Last week, a draft NHS vaccination programme suggested that everyone over the age of 18 could receive the jab by April, if vaccines are quickly authorised and 88 million doses made available. 

Andrew Bailey, the governor of the Bank of England, joined scientists in saying he could finally see “some light at the end of the tunnel”. 

However, in the hours after the announcement, AstraZeneca shares fell by roughly 4 per cent, after its headline results of 70 per cent efficacy compared unfavourably with those reported by Pfizer/BioNTech and Moderna, which both achieved efficacy of approximately 95 per cent.

On Monday AstraZeneca said it would hold talks with the Food and Drug Administration in the US to change the design of its trial, to allow the more effective regimes to be examined.