The tension between the European Union and Britain over supplies of vaccines is not the fault of either side, but is due to AstraZeneca's failure to deliver the doses it contracted to supply, an EU official has said.

"The UK is not to blame. The EU is not to blame. It's AstraZenca," the official said. "AstraZeneca has to deliver doses to its EU customers."

The official also said the EU has not formally blocked the shipment of AstraZeneca vaccines from a Netherlands factory to the UK because no export request has been made but such a request would not be approved.

Earlier, another EU official said the European Union is rebuffing British government calls to ship AstraZeneca Covid-19 vaccines produced in a factory in the Netherlands.

Britain has so far administered many more vaccines than EU countries in proportion to the population.

"The Brits are insisting that the Halix plant in the Netherlands must deliver the drug substance produced there to them. That doesn't work," the official told Reuters.

The Leiden-based plant, which is run by sub-contractor Halix, is listed as a supplier of vaccines in both the contracts that AstraZeneca has signed with Britain and with the European Union.

"What is produced in Halix has to go to the EU," the official added.

Britain has insisted that contracts must be respected.

"The European Commission will know that the rest of the world is looking at the commission, about how it conducts itself on this, and if contracts get broken, and undertakings, that is a very damaging thing to happen for a trading bloc that prides itself on the rules of law," said British Defence Minister Ben Wallace.

The EU official said the bloc was not breaking any contract.

British Prime Minister Boris Johnson is expected to ask European leaders this week to dismiss any proposals to block coronavirus vaccine exports to the UK.

Mr Johnson is expected to contact his EU counterparts ahead of a virtual summit on Thursday where European leaders are due to consider the matter, The Financial Times reported.

UK government sources said Mr Johnson spoke to Commission President Ursula von der Leyen, along with Dutch and Belgian prime ministers Mark Rutte and Alexander De Croo last week.

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The EU threatened last week to block exports of Covid-19 vaccines to Britain to safeguard scarce doses for its own citizens, with Ms von der Leyen saying the epidemiological situation was worsening.

AstraZeneca has not yet sought approval in the EU for Halix, but the official and a second EU source said the request was on its way.

Without regulatory approval, vaccines produced at Halix cannot be used in the EU.

An internal AstraZeneca document seen by Reuters shows that the company expects EU approval on 25 March.

AstraZeneca has declined to comment on the amount of vaccines that are currently stockpiled at Halix.

The EU official said the factory had already produced shots, but was not able to quantify the output.

Under the EU contract with AstraZeneca, vaccines must be produced before approval and be delivered immediately afterwards.

Two factories in Britain run by Oxford Biomedica and Cobra Biologics are also listed as suppliers to the EU in the contract with AstraZeneca, but no vaccine has so far been shipped from Britain to the EU, despite Brussels' earlier requests.

Officials have said that Cobra is not fully operational.

AstraZeneca told EU officials that the UK is using a clause in its supply contract that prevents export of its vaccines until the British market is fully served, EU officials said.

AstraZeneca vaccine 79% effective in US trial

The Oxford/AstraZeneca vaccine is 79% effective at preventing Covid-19 and offers 100% protection against severe disease, new data from a US-led trial has shown.

The jab, which has been the subject of controversy over concerns about links to very rare blood clots, is also 100% effective at keeping people out of hospital with severe illness.

Some 32,449 people across all age groups took part in the phase three trial in the US, Chile and Peru, with a total of 141 cases of symptomatic Covid-19 reported.

The results showed that among people aged 65 and over, there was 80% protection against developing Covid-19.

The degree of effectiveness for the jab was even higher than observed in the Oxford-led clinical trials.

An independent data safety monitoring board (DSMB) also identified no safety concerns related to the vaccine.

It conducted a specific review of thrombotic events, as well as cerebral venous sinus thrombosis (CVST) - the specific type of brain blood clot that has been troubling Europe.

The DSMB found no increased risk of thrombosis among the 21,583 participants receiving at least one dose of the vaccine. The specific search for CVST found no cases in this trial.

Andrew Pollard, professor of paediatric infection and immunity, and lead investigator of the Oxford University trial of the vaccine, said: "These results are great news as they show the remarkable efficacy of the vaccine in a new population and are consistent with the results from Oxford-led trials.

"We can expect strong impact against Covid-19 across all ages and for people of all different backgrounds from widespread use of the vaccine."

Poll suggests slump in trust in AstraZeneca vaccine

Meanwhile, a new poll suggests that trust in the vaccine in many European countries has plunged following controversy around the jab in recent weeks.

The survey by British pollsters YouGov carried out in the week to 18 March found a majority of people in the largest European Union member states, including Germany, France, Spain, and Italy, now see the inoculation as unsafe.

However, views towards the AstraZeneca jab remain overwhelmingly positive in Britain, where two-thirds of respondents said it is safe, compared with just 9% believing it is not.

Earlier this month several EU countries, including Ireland, suspended its use, pending a review by the European Medicines Agency (EMA) after dozens of isolated cases of blood clots and brain haemorrhages.

The EMA last week declared the AstraZeneca jab "safe and effective", with the World Health Organization and the UK's medicines regulator issuing similar endorsements.

That led European countries to resume administering the vaccine, but not before severely damaging confidence in it.

In its survey, YouGov found 55% of Germans now say the vaccine is unsafe, a 15-point rise in a month, compared with 32% viewing it as safe.

In already more vaccine-hesitant France, 61% of respondents said it was unsafe, while Italy and Spain both saw a 27-point monthly jump in the number of people saying they did not trust the jab.

No similar increases in safety fears were seen for rival vaccines by Pfizer-BioNTech and Moderna, YouGov said.

The pollsters interviewed 2,024 Germans, and around 1,000 adults in each of the other countries.