BRUSSELS, March 11 (Xinhua) -- The European Union (EU) authorized the use of a COVID-19 vaccine produced by Johnson &Johnson on Thursday afternoon, only hours after the European Medicines Agency (EMA) reached a positive conclusion on its safety and efficacy.

Developed by Janssen, the pharmaceutical arm of Johnson &Johnson, it is the fourth vaccine the EU has granted the conditional marketing authorization, after those of BioNTech-Pfizer, Moderna and AstraZeneca.

EMA recommended the use of the Janssen vaccine among people over 18 years of age. It is also the first COVID-19 vaccine which can be used in a single dose, said EMA's Executive Director Emer Cooke.

Results from a clinical trial involving 44,000 people in the United States, South Africa and Latin American countries showed that the vaccine had a 67 percent efficacy, according to EMA.

"More safe and effective vaccines are coming to the market," said European Commission President Ursula von der Leyen on Twitter. "With the number of doses we ordered, we could vaccinate up to 200 million people in the EU."

The strengthened vaccine portfolio injected hope in the implementation of the EU's vaccination programs, which aim at inoculating 70 percent of its adult population by Sept. 21.

However, the road to the ambitious target is not without obstacles. One bottleneck is the limited vaccine production that has resulted in supply shortfalls and vaccine protectionism.

On Thursday, the EU extended its transparency and authorization mechanism for vaccine exports until the end of June. The extention was announced following "persistent delays in some of the deliveries of vaccines to the EU," said the commission.

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.

"This export authorization mechanism continues to apply only to exports from companies with which the EU has Advance Purchase Agreements. These Advance Purchase Agreements commit the vaccine producers to deliver to EU Member States a pre-agreed number of vaccines," said European Commission Spokesperson Miriam Garcia Ferrer.

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 Ferrer.

What makes the matter worse is the suspension of the inoculation of AstraZeneca vaccine by Danish authorities on Thursday following reports of recently vaccinated people experiencing blood clots. Some EU members have halted a specific batch of the vaccine after Austria initially reported cases of blood clots after vaccination using doses from the batch.

The European Commission said on Thursday that the AstraZeneca vaccines is still safe to use, adding that the EMA has adopted a preliminary review on the case in Austria "where they said there is no specific indication that the vaccination led to these conditions."