The European Commission refused to comment directly Friday on a leak of how much it would pay for COVID-19 vaccine doses, stressing confidentiality clauses with the companies involved.

Spokesmen for the EU executive were questioned about the information revealed Thursday in a tweet by a junior minister in Belgium's government that gave a cost breakdown of six vaccines in the Commission's portfolio.

The tweet was deleted shortly afterwards, but screengrabs of it were quickly posted on social media.

"We cannot say anything about this," one of the spokesmen, Stefan de Keersmaecker, told journalists, without denying the tweet's accuracy.

"Everything that has to do with information of prices of vaccines is covered by confidentiality. This is something which is very important," he said.

The Commission's chief spokesperson Eric Mamer added that secrecy over the prices paid was "contractually required" by the companies supplying the vaccines. 

Without that, he said, "we would not have had those contracts".

The tweeted information showed prices for six of the seven vaccines the Commission has bought. 

Price per dose ranged from €1.78 for a potential vaccine by British-Swedish company AstraZeneca, to €14.70 for the most expensive jab, from the US firm Moderna. 

The price for the Pfizer/BioNTech vaccine - so far the only one to receive authorisation, for use in the UK, the US and probably next week the EU - was given as €12 a dose. 

Contacted by AFP for comment, Pfizer did not immediately respond.

De Keermaecker said that the confidentiality clause in the contracts was not only to protect "sensitive business information" but also "the public's interest".

"If all this sensitive information were to be made public, this would weaken the position of the negotiators of the Commission and the member states involved in these negotiations," he said.

The tweeted information gave the following price per dose for each vaccine maker, in either euros or US dollars according to the respective contract:

1. AstraZeneca: €1.78 

2. Johnson & Johnson: €6.94

3. Sanofi/GlaxoSmithKline: €7.56 

4. Pfizer/BioNTech: €12.00 

5. Curevac: €10

6. Moderna: €14.70

Separately, on Thursday, the European Commission announced a seventh contract with US biotech company Novavax for 100 million doses, the price of which was not given under the confidentiality agreement.