What started with Austrian regulators suspending use of one batch of Astrazeneca plc’s COVID-19 vaccine has expanded into precautionary holds in Denmark and other EU countries as PRAC, the EMA’s safety committee, investigates whether blood clots, which have resulted in at least two deaths in Europe, are connected to the vaccine.

There’s “currently no indication that vaccination has caused these conditions, which are not listed as side effects with this vaccine,” the EMA said. Results from the investigation so far indicate the vaccine’s benefits still outweigh its risks, and it can be administered even as the investigation continues, the regulator added.

As of March 10, Eudravigilance showed 30 cases of thromboembolic events had been reported among the 5 million people vaccinated with the Astrazeneca vaccine in the European Economic Area.

That’s about 0.0006% – much lower than the 0.1% to 0.2%, or one to two in 1,000, seen in the broader population. Venous thrombosis is relatively common, as it can occur for a variety of reasons and the lead up to an event often is undetected, according to Jon Gibbins, director of the Institute for Cardiovascular and Metabolic Research at the University of Reading in the U.K.

“If there is any association between the vaccine and clotting, the risk is likely to be very low indeed,” he told the Science Media Centre. On the other hand, he noted, “there is good emerging evidence that severe COVID-19 infection itself can cause harmful blood clots through the body’s own immune system, and so it may be the case that people are more at risk of clotting from not being immunized.”

Adam Finn, professor of pediatrics at the University of Bristol in the U.K., noted that, at the moment, “there is no sign, anywhere, including the U.K. where very large numbers of doses have now been given, that blood clot-related illnesses are happening any more frequently than usual. That’s reassuring because it means either that the vaccine doesn’t cause blood clots at all or, at the very worst, that it’s an extremely rare event.”

However, he noted, “when something bad happens after you have had a vaccination, it’s natural enough to wonder whether the vaccine was the cause.” And when large numbers of people are being vaccinated over a short period of time, he said, “a certain number of unexpected and unusual illnesses are going to happen in the period following vaccination by chance. If a pattern emerges, then suspicion rises that there may be a causal link, especially if there’s a mechanism that could explain how the two events could lead one to the other.”

Austrian regulators sounded the alarm about the one batch of Astrazeneca’s vaccine after a person was diagnosed with multiple thrombosis and died 10 days after vaccination and another person was hospitalized with pulmonary embolism. As of March 9, two other reports of thromboembolic events had been received in connection with the batch, which comprised 1 million doses that were delivered to 17 EU countries, according to the EMA.

Subsequently, Estonia, Latvia, Lithuania and Luxembourg suspended the batch as a precautionary measure. Meanwhile, in Denmark and other EU countries, health officials paused their entire vaccination campaign with the Astrazeneca vaccine. The hold in Denmark followed the report of a vaccinated person dying there after developing blood clots, according to the EMA.

In response to the incidents, the PRAC is reviewing all cases of thromboembolic events, and other conditions related to blood clots, reported post-vaccination with the Astrazeneca vaccine. It also is conducting a batch quality investigation of the vaccine batch suspended in Austria, “although a quality defect is considered unlikely at this stage,” the EMA said.

“We will need to see the detailed data in order to assess this properly,” Gibbons said, “but at this point we also need to be careful not to cause unwarranted panic or resistance to vaccination.”