The EU's drug regulator said Thursday the AstraZeneca coronavirus vaccine was "safe and effective" and was not linked to an increased risk of blood clots.

The European Medicines Agency (EMA) however said that after an investigation it could not "definitively" rule out a connection to a particularly rare type of blood disorder.

Around a dozen countries had suspended the use of the jab and were awaiting the outcome of the probe by the Amsterdam-based watchdog's safety committee.

"The committee has come to a clear scientific conclusion: this is a safe and effective vaccine," EMA Executive Director Emer Cooke told an online press conference.

"Its benefits in protecting people from COVID-19 with the associated risks of death and hospitalization outweigh the possible risks," she said.

"The committee also concluded that the vaccine is not associated with an increase in the overall risk of thromboembolic events or blood clots."

Cooke added that: "If it was me I would be vaccinated tomorrow."

The EMA said however that further investigation was needed into a number of rare cases linked to the AstraZeneca jab that involved low levels of blood platelets - cells that help clotting.

It was particularly looking at cases of cerebral venous thrombosis, or blood clots in the vessels draining blood from the brain.

"During the investigation and review we began to see a small number of cases of rare and unusual but very serious clotting disorder and this then triggered a more focused review," said Cooke.

"Based on the evidence available, and after days of in-depth analysis of lab results, clinical reports, autopsy reports and further information from the clinical trials, we still cannot rule out definitively a link between these cases and the vaccine."

The EMA had recommended adding a warning to product information with the AstraZeneca shot.

This would draw attention to the "possible rare conditions" to help patients and healthcare professionals "stop and mitigate any possible side effects," said Cooke.

There had been 469 reports of blood clotting among 20 million people who had been vaccinated in the European Economic Area (EU, Norway, Iceland and Liechtenstein) and Britain, the EMA said in a separate statement.

In all 191 were from the EEA, it said.

Comparing this figure to cases in the general population, it had found that there was no increase in overall risk of blood clot, it said.

It found just 25 cases of the rare condition involving low levels of platelets. Almost all of these were in women under 55.

"However, in younger patients there remain some concerns, related in particular to these rare cases," it added.

There was meanwhile "no evidence of a problem" linked to specific batches of the vaccine - one of the early lines of inquiry, the EMA said.

Nor was there any suspected connection with particular vaccine factories.

© Agence France-Presse