AstraZeneca's boss expressed confidence Thursday that his company can make up a huge shortfall in Covid-19 vaccine doses promised to the EU, in a grilling by MEPs alongside rival CEOs.

Pascal Soriot, speaking by videolink, told the European Parliament the delivery deficit -- equal to around 60 percent of the 100 million doses meant to be supplied between January and March -- was because of low production yields in the EU factories making AstraZeneca's vaccine.

But, he said, "we hope that by Q2 (the April-May-June second quarter), we can catch up to where we would like to be".

The undersupply torpedoed the EU's plan to use AstraZeneca as its main first vaccine in the first quarter.

But EU officials say ramped up deliveries from April by the Anglo-Swedish company, and by BioNTech/Pfizer and Moderna, mean that the 27-nation bloc remains on track to reach its goal of fully vaccinating 70 percent of adults by mid-September.

EU leaders held a videosummit on Thursday to look at ways to further boost access to doses, as well as discussing the launch of a vaccine certificate system to be used initially as a medical record rather than a "vaccine passport" allowing easier travel.

Soriot, a Frenchman excoriated by Brussels for AstraZeneca's failure to live up to its delivery promises while continuing to fully supply Britain, blamed the EU shortfall on "a lower yield than we expected" in European factories.

He said that doses were shipped out as fast as they were made so there was no inventory to act as a supply buffer.

"We are very confident that the yield will increase across the network" and other AstraZeneca plants would be tapped to improve the EU's supply after April so that "we actually catch up to the volume that we had projected originally to produce," he said.

Soriot said those factories "will be a variety of locations around the world, including the United States and some others".

No supply from UK

Soriot said an AstraZeneca plant in the Netherlands "originally reserved by the UK" for its supply would be part of that effort, but he appeared to rule out the company's UK factories diverting some of their production to the EU.

"The UK is geared to supply 65 million people who live in the UK. The European community has about 450 million people I believe. So you know, the UK, even if we took the entire supply of the UK would not make a huge difference to the European community," he said.

The head of the European Parliament committee questioning Soriot, Pascal Canfin, was sceptical that AstraZeneca would make good.

Soriot, he tweeted, "was not able… to confirm the expected delivery of 180 million doses in Europe in the second quarter," and he warned of "new tensions" with the company if that turned out to be the case.

The European Commission, suspicious that some AstraZeneca vaccine produced in the EU had been sent to Britain, on January 29 launched a scheme to monitor -- and if necessary, block -- exports of Covid vaccines from the European Union.

Franz-Werner Haas, the CEO of German company CureVac which is hoping its vaccine will soon be approved for use in the EU, said his company fell foul of the new system.

"This has been hitting us somehow because we couldn't get clinical trial material out to Latin America," he said. "We lost a week then to ship the material, which was totally unnecessary."

He added, though, that the commission and the German government did give "immediate support" to resolve the blocage.