BRUSSELS, March 11 (Xinhua) -- The transparency and authorization mechanism for exports of COVID-19 vaccines out of the European Union (EU) has been extended until the end of June, announced the European Commission on Thursday.

The extension was announced following "persistent delays in some of the deliveries of vaccines to the EU," said the commission in a press release.

Put in place on Jan. 30, the mechanism was originally due to expire on Friday. It was launched when the EU argued that some pharmaceutical companies failed to honor their commitments to deliver the promised amount of doses to the EU countries but still could conduct exports from EU-based facilities.

"We expect companies with which we have signed a contract to fulfil their obligations towards EU citizens. The EU exports very significant volumes of COVID-19 vaccines, true to our commitment to global solidarity. Yet, not all companies are honoring their agreements with the EU despite having received a down payment to enable sufficient production," said EU Commissioner for Health and Food Safety Stella Kyriakides.

Backed by the commission, Italy banned the export of 250,000 doses of AstraZeneca vaccine destined to Australia in early March. This was so far the only case in which the export request was refused.

Since the mechanism was put in place, 249 export requests towards 31 different countries and regions were approved. In total, more than 34 million doses have been exported "as they did not threaten the contractual engagements between the EU and the vaccine producers," said the commission.

The European Commission signed advance purchase agreements with six companies on behalf of its member states. So far, four out of the six candidates have been granted conditional market authorization in the bloc, and the contracted amount is about 1.6 billion doses.

The EU has rolled out vaccination programs aiming at inoculating 70 percent of its adult population by Sept. 21.

Meanwhile, 263 candidate vaccines are still being developed worldwide -- 81 of them in clinical trials -- in countries including Germany, China, Russia, Britain, and the United States, according to information released by the World Health Organization on March 9.