The new criteria have been introduced in the midst of an ongoing dispute between the European Union and the United Kingdom on vaccines - specifically concerning the AstraZeneca vaccine - and the United Kingdom may fall foul of both reciprocity and proportionality principles.

British-Swedish drugmaker AstraZeneca on Wednesday revised down by three percentage points the effectiveness of its Covid vaccine after American authorities raised concerns that results reported from its United States trial were outdated.

Asked by correspondents about safety concerns over the AstraZeneca Covid vaccine and others distributed through global vaccine access mechanism Covax, he said the Pharmacovigilance Risk Assessment Committee, the European Medicines Agency's committee responsible for assessing and monitoring the safety of human medicines, has concluded that the vaccine's benefits far outweigh its potential risks, and that cases of blood clots are exceedingly rare.

The US National Institutes of Health then issued a highly unusual statement asking AstraZeneca to work with the panel and issue a new press release. "We look forward to filing our regulatory submission for Emergency Use Authorization in the U.S. and preparing for the rollout of millions of doses across America", Mene Pangalos, executive vice president for biopharmaceuticals research for the company, said in a statement. "There are 14 additional possible or probable cases to be adjudicated so the total number of cases and the point estimate may fluctuate slightly", the company said.

The vaccine appears especially protective against the worst outcomes, with no severe illnesses or hospitalisations among vaccinated study volunteers compared to eight severe cases among those given dummy shots, the company said.

The firm said its vaccine also has 100% efficacy against severe or critical disease and hospitalisation due to coronavirus and is 85% effective against symptomatic Covid-19 in patients aged 65 years and over.

A health worker prepares a dose of the AstraZeneca vaccine, during a mass vaccination campaign at San Pedro Hospital, in Logrono, northern Spain.

"We in this country don't believe in blockades of any kind of vaccines or vaccine materials", he said. As with Monday's data, the company has released them via news release and not in a peer-reviewed report or as a formal submission for US Food and Drug Administration review.

Previous studies have turned up inconsistent data about its effectiveness, and then last week a scare about blood clots had some countries temporarily pausing inoculations.

The European Medicines Agency has since said there's no evidence the vaccine can cause blood clots.

The Oxford-AstraZeneca vaccine has been the leading choice in the developing world because of its low cost and simple storage requirements.

The cells then produce that protein on their surface, training the immune system should it encounter the real virus.