As a growing list of countries halts administration of the Oxford University-AstraZeneca coronavirus vaccine amid as-yet-unsubstantiated concerns about fatal blood clots, the World Health Organization has offered assurances that the vaccine’s safety is being investigated and that there is no cause for alarm.

“More than 335 million doses of COVID-19 vaccines have been administered globally so far, and no deaths have been found to have been caused by COVID-19 vaccines,” WHO Director General Tedros Adhanom Ghebreyesus said at a briefing Friday in Geneva.

“We should continue immunization until we have clarified the causal relationship,” said Mariangela Simao, WHO’s assistant director general, adding that regulators are investigating and that the body is expected to release a statement on the matter by the middle of next week.

“People die every day,” Simao said. “There will be people who are immunized who die of other causes.”

WHO Chief Scientist Soumya Swaminathan said the number of deaths and adverse reactions would need to be compared with those among the general population. “It could be completely unrelated, but it’s reported because people are keeping a close watch,” she said.

Italy, Romania and Thailand joined at least eight other European countries this week in suspending the injections either from specific batches or as part of a total freeze, citing the potential adverse events, while the European Medicines Agency (EMA) and the WHO investigate.

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Concern about the vaccine, which was developed by researchers at the famed English university and brought to market by the British-Swedish pharmaceutical giant, has been linked to a case in Austria of a woman who was diagnosed with blood clots and died 10 days after being vaccinated.

Fatal blood clots among people who had received the vaccine have also been reported in Denmark and Italy, though there is no evidence the deaths were caused by the vaccine.

No vaccine authorized for use in Europe or the United States has been directly linked to a death, even in clinical trials. AstraZeneca is not yet authorized in the United States, but a decision is expected next month when the results of the first large-scale U.S. human trials come in.

In a statement this week, AstraZeneca said that “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.”

The EMA, the European Union’s drug regulator, said Friday it is investigating the reports of blood clots following AstraZeneca immunization but emphasized there is “no indication that vaccination has caused these conditions, which are not listed as side effects with this vaccine.”

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“The vaccine’s benefits continue to outweigh its risks,” the EMA said in a statement.

Simao said investigations are focused on two batches of AstraZeneca that were manufactured in Europe and do not affect the rollout of the vaccine through the COVAX Facility, an international effort to distribute vaccines that uses doses manufactured in India and South Korea.

The decision to halt the use of AstraZeneca in some European countries comes as another blow to the continent’s vaccine rollout, which has been considerably slower than in the United States and neighboring Britain. E.U. leaders have criticized the manufacturer over delays.

The AstraZeneca vaccine is one of four vaccines approved in the 27-nation European Union, and it had been viewed as an important source of supply for global vaccine uptake as it is cheaper and easier to store and distribute.

But in some parts of Europe, there has been lower uptake of the vaccine than of its rivals, in part because some regulators initially hesitated over it, given the limited early data on its efficacy among people over 65.

While several European countries have suspended the rollout of some batches of the vaccine, they say it is only a precautionary measure. On Thursday, Italian authorities said no link had been established between the vaccine and subsequent “serious adverse events.”

Thailand announced Friday that it is delaying its immunization program using the AstraZeneca vaccine, citing the European reports. The campaign was supposed to begin Friday, with the prime minister receiving the first shot.

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Cunningham reported from Istanbul.