Denmark, Norway, and Iceland on Thursday suspended use of AstraZeneca's COVID-19 vaccine.
Several cases of blood clots were found among vaccinated people, health authorities in the countries said.
European regulators said it wasn't clear whether the blood clots were linked to the vaccine.
Denmark, Norway, and Iceland have paused use of AstraZeneca's COVID-19 vaccine while investigators look into several cases of blood clots among vaccinated people.
One of these cases was related to a death in Denmark, it said. This follows Austrian authorities saying Sunday that a 49-year-old woman had died as a result of severe coagulation disorders after taking the shot.
Magnus Heunicke, the Danish health minister, said it wasn't clear whether the blood clots were linked to the vaccine.
Around three hours after Denmark's announcement, Norwegian officials said they would also suspend use of the AstraZeneca vaccine.
The Financial Times first reported that Iceland had also suspended the use of the vaccine, and Kjartan Njálsson, assistant to the director of health in Iceland, told CNN that officials were awaiting advice from the European Medicines Agency (EMA).
Both the Danish and Norwegian health ministries said that there was good evidence the vaccine, developed by AstraZeneca and Oxford University, was safe and effective but that they must react to reports of possible serious side effects.
People who received their first dose of AstraZeneca's vaccine would have to wait for their second dose during the suspension, Denmark's health authority said.
It added that it was waiting for the results of an investigation by the EMA.
The EMA said that in the European Economic Area, 30 cases of blood clots had been reported out of close to 5 million people vaccinated. UK experts said the proportion of vaccinated people with blood clots was not significantly higher than in the general population.
The vaccine has been granted conditional marketing authorization or emergency use in more than 50 countries, including the UK and across the European Union. It has not yet been issued emergency-use authorization in the US.
AstraZeneca says vaccine is "generally well tolerated"
In a statement to Insider, an AstraZeneca representative said that the safety of its vaccine had been "extensively studied" and that data showed the shot was "generally well tolerated."
"Patient safety is the highest priority for AstraZeneca," the person said. "Regulators have clear and stringent efficacy and safety standards for the approval of any new medicine, and that includes COVID-19 Vaccine AstraZeneca.
"The safety of the vaccine has been extensively studied in Phase III clinical trials and Peer-reviewed data confirms the vaccine has been generally well tolerated."
Stephen Evans, Professor of Pharmacoepidemiology, London School of Hygiene & Tropical Medicine, said in a statement that the approach was "super cautious."
"Since we know with great certainty that the vaccine prevents COVID-19, and we are almost totally uncertain that the vaccine can have caused this problem, the risk and benefit balance is still very much in favour of the vaccine in my view," he said.
Evans said that it was difficult to distinguish between coincidence and the vaccine's side effects.
"This is especially true when we know that COVID-19 disease is very strongly associated with blood clotting and there have been hundreds if not many thousands of deaths caused by blood clotting as a result of COVID-19 disease," he said.
Denmark and Norway have also approved both the Pfizer-BioNTech and Moderna coronavirus vaccines.
Denmark's health authority said that if it permanently suspended AstraZeneca's vaccine, it would push back its vaccination plan by about four weeks. Norway's health authority also said the temporary suspension would delay the county's vaccine rollout.
Norway's health authority said that so far 121,820 people have been given the AstraZeneca vaccine and that these people should not be worried.
5 EU countries suspend one specific batch of AstraZeneca's vaccine
It's not clear whether the decision of Denmark's health authority was based on the same EMA data that prompted five other EU countries - Austria, Estonia, Lithuania, Luxembourg, and Latvia - on Sunday to suspend use of a specific batch of AstraZeneca's COVID-19 vaccine as a precaution following the death of a vaccinated person.
The batch consisted of 1 million doses that were distributed among 17 EU countries.
Insider contacted the Danish health authority for clarification, and a spokesperson said that they were "not at liberty to inform on the data that we receive from other authorities, national or international."
Italy on Thursday suspended a batch, too.
In Europe, one person had been diagnosed with multiple blood clots and died 10 days after vaccination, the EMA said Wednesday. Another had a blood clot in their lung, and was hospitalized.
The EMA noted two other "thrombotic events" in people who had received the vaccines.
"There is currently no indication that vaccination has caused these conditions, which are not listed as side effects with this vaccine," the EMA said while adding that it was investigating.
The Medicines and Healthcare Products Regulatory Agency (MHRA), the UK's vaccine regulator, said people should still take the vaccine, and that blood clots "occur naturally and are not uncommon."
Dr Phil Bryan, MHRA vaccines safety lead, said in a statement: "Vaccine safety is of paramount importance and we continually monitor the safety of vaccines to ensure that the benefits outweigh any potential risks. It has not been confirmed that the report of a blood clot, in Denmark, was caused by the COVID-19 Vaccine AstraZeneca.
"The Danish authorities' action to temporarily suspend use of the vaccine is precautionary whilst they investigate. Blood clots can occur naturally and are not uncommon.
"More than 11 million doses of the COVID-19 Vaccine AstraZeneca vaccine have now been administered across the UK. Reports of blood clots received so far are not greater than the number that would have occurred naturally in the vaccinated population."
A spokesperson for British Prime Minister Boris Johnson told AFP the vaccine was "both safe and effective."
Shares in AstraZeneca slipped by more than 2% after Denmark announced its plan to halt the vaccine.
As Insider's Marianne Guenot reported early in March, AstraZeneca's vaccine has become Europe's least favorite shot. Europeans are turning down the AstraZeneca vaccine, and both AstraZeneca and EU leaders have made mistakes that sapped public confidence.
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