The European Medicines Agency (EMA) is expected to complete its safety review of issues relating to the AstraZeneca vaccine this week.
It follows the temporary suspension of the use of the vaccine by a growing number of countries in Europe, including Ireland.
The EMA said its safety committee will review the information today on the AstraZeneca vaccine and will hold an extraordinary meeting on Thursday to conclude the issues.
It said that while its investigation is ongoing, it currently remains of the view that the benefit of the AstraZeneca vaccine in preventing Covid-19 outweighs the risks of side-effects.
Many thousands of people develop blood clots each year for different reasons, it said.
The agency said that the number of events overall in vaccinated people seems not to be higher than that seen in the general population.
Yesterday, Germany, France, Italy and Spain joined a number of other European countries in Europe, pausing the use of the AstraZeneca vaccine, while reports on safety issues are assessed.
Ireland temporarily suspended the use of the vaccine yesterday as a precautionary measure.
It means around 30,000 vaccinations here for health staff and those at very high risk may be delayed.
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The head of the International Health and Tropical Medicine at the RCSI, Professor Sam McConkey has said the EMA will be looking for Norweigan authorities to give a detailed dossier of each of the four cases.
Speaking on RTÉ's Liveline, he said they will be looking to see if there were clots, how serious they were, and what type of clots they were.
He said they will also be asking if the rate of clotting is higher in those who have received an AstraZeneca vaccine than in the average population.
AstraZeneca itself also has data on millions of people who have received the vaccine, Prof McConkey said, and they can provide that so the EMA can make an independent review.
He said clots are "unfortunately a common thing" that can happen by chance.
"So what the EMA will determine from all this data is, one is there a higher rate of clotting in folk after the vaccine than in the first week or two before?
"And secondly how big is that rate, because if it's only one in a million, it really doesn't matter."
Prof McConkey said pausing the administration of AstraZeneca vaccines is the sign of a "cautious, well-run, prudent programme" and he thinks it is "reassuring" for people to see that we have a Government decision making system that is cautious and wants to see all the facts before proceeding.
Today the World Health Organization said its advisory committee was assessing the issue and that there was no evidence that any health incidents were caused by the vaccine.
It said it was important that vaccination campaigns continue, in order to save lives and stem severe disease from the virus.
WHO chief Tedros Adhanom Ghebreyesus said the agency's vaccine safety experts would meet tomorrow to discuss the AstraZeneca jab.
"WHO's advisory committee on vaccine safety has been reviewing the available data, is in close contact with the European Medicines Agency, and will meet tomorrow," he told a press briefing.
