• Austria, Latvia, the Czech Republic, Bulgaria, Croatia and Slovenia wrote to the European Commission to complain that jabs are not being delivered on a proportionate basis.
• Their complaint follows news that AstraZeneca will not meet its delivery targets in the coming months.
• The European Commission has argued that distribution is a "transparent process" and it was the member states' decision to add flexibility to the allocation of inoculations.

LONDON — Six members of the European Union have raised concerns over how the bloc is distributing Covid-19 vaccines, after AstraZeneca cut its delivery targets once again.

Austria, Latvia, the Czech Republic, Bulgaria, Croatia and Slovenia wrote to the European Commission on Saturday to complain that jabs are not being delivered on a proportionate basis among the 27 countries which make up the European Union.

"If this system were to carry on, it would continue creating and exacerbating huge disparities among member states by this summer," the heads of state wrote in a letter obtained by CNBC.

It was initially agreed that vaccines bought by the EU would be handed out proportionately to the size of a country's population. But some countries introduced flexibility into the system so they could opt for more of a specific vaccine based on price and maintenance conditions.

The European Commission responded to the letter by saying the distribution is a "transparent process" and that it was the member states' decision to introduce this flexibility.

"Under this system, if a member state decides not to take up its pro rata allocation, the doses are redistributed among the other interested Member States," the commission said in a statement.

According to media reports, Bulgaria, for instance, opted to receive fewer Pfizer and BioNTech shots, the most expensive of the vaccines, and more of the shot developed by AstraZeneca and the University of Oxford. As a result, other EU countries were able to buy the excess Pfizer and BioNTech vaccines.

The Bulgarian government was not immediately available for comment when contacted by CNBC on Monday.

Bulgaria and the other signatories are among the EU nations with the lowest number of vaccines received so far, according to data from the European Centre for Disease Prevention and Control. 

They are worried that without any changes, some EU nations "would be able to reach herd immunity in a few weeks while others would lag far behind," they said in their letter.

Their complaint follows news that AstraZeneca will not meet its delivery targets in the coming months. The Swedish-British pharmaceutical firm confirmed to CNBC on Monday that it will deliver 30 million doses to the EU by the end of the first quarter and another 70 million doses during the second quarter.

These numbers are below what the bloc was expecting to receive.

"Why do they come up with this now, knowing that Austria is a member of the steering board, like the 26 other member states, and has been informed of the previous allocations like the others," a European official, who did not want to be named due to the sensitivity of the issue, told CNBC on Sunday.

This comment suggests that the six countries could have dealt with the issue internally, rather than writing a letter and making it public.

Pascal Donohoe, Ireland's finance minister, told CNBC on Monday that if it wasn't for the European Commission's work overseeing the distribution of vaccines, the issues "would have been greater."

It is expected to be discussed at the next European Summit, later this month.