Brussels/Vienna - Five EU prime ministers, including Slovenia's Janez Janša, have addressed a letter to Brussels calling for a summit on Covid-19 vaccines. The call comes after Austrian Chancellor Sebastian Kurz strongly criticised the distribution of vaccines among EU member states, saying some EU countries were receiving more vaccines than others.

The letter was sent to European Council President Charles Michel and European Commission President Ursula von der Leyen on Friday, and was released on Saturday.

It was also signed by the prime ministers of the Czech Republic, Latvia and Bulgaria.

Kurz accused on Friday some EU members of concluding separate contracts with vaccine producers, arguing this led to vaccines being unevenly distributed among EU members.

The latter says that in recent days, "we have discovered that deliveries of vaccine doses by pharma companies to individual EU member states are not being implemented on an equal basis following the pro rata population key" agreed by EU leaders.

Should this continue, there will be "huge disparities among members", with some members unable to reach herd immunity in a few weeks while others would lag far behind.

The letter also says that "this would be contrary not only to our agreement but also to the spirit of European solidarity".

Kurz did not name any member states in particular during his news conference, but said that by end of June, Malta would have received three times more vaccines than Bulgaria and the Netherlands double Croatia's amount.

Janša spoke with Kurz and also retweeted his related tweets yesterday, adding that "this is important" and that "unequal treatment is totally unacceptable. The EU has enough power and resources to secure vaccines for all citizens older than 18 in the first half of 2021."

The European Commission meanwhile denied the allegations yesterday, and so has the Austrian Health Ministry, which added that the talks and the distribution of vaccines were "balanced and transparent".

An Austrian Health Ministry official said that all EU members had an option of buying the vaccines which a certain member state had not used or simply renounced, for instance due to costs involved.

Ines Stilling also explained that at last summer's EU talks, all member states had an opportunity to say what an amount of a certain vaccine they would like to receive. The pro rata population key was followed in principle, yet countries decided to purchase very different amounts of vaccines.

The Commission today issued a written statement on the methodology used to determine the allocation of doses of vaccines under the advance purchase agreements, saying it was transparent.

It said it agreed that the most equitable solution is on the basis of a pro rata of population of each member state, a solution it had proposed for all advance purchase agreements.

However, member states decided to depart from the Commission's proposal by adding a flexibility which allows agreeing on a different distribution of doses, taking into account the epidemiological situation and the vaccination needs of each country.

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 statement says, adding that "it would be up to the member states to find an agreement if they wished to return to the pro rata basis".