Germany, France, Italy and Spain suspended use of AstraZeneca’s coronavirus vaccine on Monday over latest reports of blood clots in people who were administered the drug.
World Health Organisation (WHO) chief scientist Dr Soumya Swaminathan later on Monday called for calm and said countries should continue AstraZeneca immunisations
The string of suspensions is the latest blow to Europe's relationship with the pharmaceutical company after a row earlier this year over supply shortages.
The German Health Ministry said its decision was taken as a “precaution” and on the advice of national vaccine regulator the Paul Ehrlich Institute, which called for further investigation.
The ministry said the European Medicines Agency (Ema) would decide "whether and how the new information will affect the authorisation of the vaccine".
The reported blood clots involved cerebral veins, but the ministry didn't specify where or when the incidents occurred.
French President Emmanuel Macron said medical workers would stop administering AstraZeneca's vaccine pending an assessment from the Ema, due on Tuesday.
"The decision, which has been taken out of precaution, is to suspend vaccinating with the AstraZeneca vaccine in the hope that we can resume quickly if the EMA gives the green light," he said.
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Italy will also wait for the regulator's advice before lifting its suspension, authorities said.
Several European countries – including Ireland, Austria and the Netherlands – recently suspended use of the vaccine made by AstraZeneca and Oxford University to investigate cases of blood clots that occurred after inoculation.
Norway and Denmark halted use of the vaccine last week after one person in Denmark who had received the drug suffered a blood clot and died.
Norwegian officials reported instances of people “bleeding under the skin” and “severe cases of blood clots or brain haemorrhages in younger people” who had received the vaccine.
AstraZeneca said there was no cause for concern and that there were fewer reported thrombosis cases in vaccinated people than in the general population.
WHO chief Dr Tedros Adhanom said that the incidence of blood clots weren't necessarily linked to the vaccine and that he would be meeting with the European Medicines Agency on Tuesday.
He warned that the greatest global threat remained to be a lack of access to vaccines.
"Almost every day, I receive calls from senior political leaders around the world asking when their country will receive their vaccines through Covax," he said.
The WHO also emphasised that safety concerns raised are for AstraZeneca vaccine batches produced in Europe, not those produced for Covax in South Korea and India.
Saudi Arabia's Health Ministry said the AstraZeneca Covid-19 vaccine was safe.
Meanwhile, the UAE carried out nearly 38,000 vaccinations since Sunday, bringing the total to 6,578,881.
The UAE also reported 1,898 new cases of the coronavirus after 205,579 new tests.
The latest figures also show 2,438 recoveries and seven deaths in the Emirates.
Blood clots across the population 'normal'
In the wake of the decision to suspend the AstraZeneca vaccine in some European nations, Cardiff University's infectious disease expert Dr Andrew Freedman said that when such large numbers of people were being studied it would be normal to find blood clots across the population.
"It's unlikely to be a causal relationship," he told The National. "I expect they will resume the use of the AstraZeneca vaccine once they've had a chance to look at the cases."
Prof Stephen Evans from the London School of Hygiene and Tropical Medicine told Sky News that European authorities were acting hastily.
"What I think is happening is once there is a scare in one place, then those who are delivering the vaccine start to become anxious when they hear other countries have stopped using it," he said.
"I would very much doubt epidemiological data will show the vaccine has this problem. Certainly, the UK data does not give any indication of this problem."
AstraZeneca said it found no evidence of an increased risk of pulmonary embolism, deep-vein thrombosis or thrombocytopenia in either sex or within any specific age group, country or batch.
"About 17 million people in the EU and UK have now received our vaccine and the number of cases of blood clots reported in this group is lower than the hundreds of cases that would be expected among the general population," AstraZeneca's chief medical officer Ann Taylor said.
“The nature of the pandemic has led to increased attention in individual cases and we are going beyond the standard practices for safety monitoring of licensed medicines in reporting vaccine events to ensure public safety.”
After Ireland announced on Sunday that it was suspending use of the AstraZeneca vaccine as a “precautionary step”, the UK's medicines regulator said the available evidence “does not suggest the vaccine is the cause” of clots.
“Given the large number of doses administered and the frequency at which blood clots can occur naturally, the evidence available does not suggest the vaccine is the cause,” UK Medicines and Healthcare Products Regulatory Agency safety lead Dr Phil Bryan said.
“People should still go and get their Covid-19 vaccine when asked to do so.”
AstraZeneca became embroiled in a bitter row with European leaders earlier in the year when it announced it would not be able to supply the number of doses it promised.
Since then, EU leaders have criticised the pharmaceutical company, cast doubt on the vaccine’s efficacy and halted some exports of doses to destinations outside the bloc.