European countries have suspended use of a Covid-19 vaccine developed by AstraZeneca and the University of Oxford due to a small number of reports of blood clots—but a WHO spokesperson said "there is no evidence that the incidents are caused by the vaccine" and urged that vaccinations continue.

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Why countries are suspending use of AstraZeneca's Covid-19 vaccine

The blood clots being reported are, in most cases, venous thromboembolic events, which are relatively common in the general population. During a venous thromboembolic event, thickened clumps of blood form in a blood vessel, which can result in fatal blockages, the Wall Street Journal reports.

At least nine European countries—Denmark, France, Germany, Iceland, Ireland, Italy, the Netherlands, Norway, and Spain—have temporarily halted the use of AstraZeneca's Covid-19 vaccine, which FDA has not yet authorized for use in the United States, over a small number of reported blood clots among people who had received the vaccine.

For example, German Health Minister Jens Spahn on Monday said the country had paused its rollout of AstraZeneca's vaccine as precautionary measure. The move came after the Paul Ehrlich Institute, Germany's vaccine regulator, recommended the suspension following seven reported cases of blood clotting among people who had received the vaccine. Germany so far has administered 1.6 million doses of AstraZeneca's vaccine, the Journal reports.

In Norway, officials reported four adverse events among health workers under the age of 50 who had received the vaccine, the New York Times reports. Norwegian health authorities said most of them had developed blood clots or bleeding abnormalities, and they had low platelet counts. Two of them have died from brain hemorrhages, and the other two have been hospitalized, the Times reports.

The Times notes, however, that "none of the deaths have been fully investigated to determine whether there is any link to the shots they received."

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Despite the reports of blood clots, the European Medicines Agency (EMA), the European Union's drug regulator, and the World Health Organization (WHO) say that—at least so far—there's insufficient reason to believe the vaccine caused the blood clots, and that the vaccine's benefits continue to appear to outweigh the risks. However, EMA and WHO are investigating the reports.

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Christian Lindmeier, a WHO spokesperson, told Reuters, "As of today, there is no evidence that the incidents are caused by the vaccine, and it is important that vaccination campaigns continue so that we can save lives and stem severe disease from the virus."

EMA said it is working with AstraZeneca and health authorities to review "all the available data and clinical circumstances surrounding specific cases." EMA is expected to release its findings on the vaccine's safety and risks from a review of reported cases on Thursday.

Similarly, the United Kingdom's Medicines and Healthcare products Regulatory Agency, which became the first regulator to authorize the vaccine's use in December, advised residents to receive their shots of AstraZeneca's vaccine as planned.

The International Society on Thrombosis and Haemostasis also recommended all adults eligible for AstraZeneca's vaccine get their shot, saying the small number of reported blood clotting cases relative to the millions of vaccine doses administered doesn't suggest there's a causal link.

For its part, AstraZeneca has said the rate of reported blood clotting cases among the roughly 17 million people in the European Union and United Kingdom who've received its vaccine is lower than occurs in the general population. In addition, large-scale human clinical trials had not identified blood clotting as a vaccine-related risk.

Some experts believe the blood clotting cases reported among recipients of AstraZeneca's vaccine are coincidental and not linked to the vaccinations, the Times reports.

"There are a lot of causes of blood clotting, a lot of predisposing factors, and a lot of people who are at increased risk—and these are often also the people who are being vaccinated right now," said Mark Slifka, a vaccine researcher at Oregon Health and Science University.

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To illustrate the background risk, Stephan Moll, a hematologist and professor of medicine at the University of North Carolina, noted that an estimated 1,000 to 2,000 blood clots occur daily in the United States. If roughly 1% of the population receives a vaccine each day, then 10 to 20 blood clots would, by coincidence, occur daily in the just-vaccinated population. "Only if epidemiological data show that that rate is higher would one start to wonder about a causative relationship," Moll said.

In the United States, health officials are monitoring thromboembolic events among several types of possible adverse events as Covid-19 vaccines are more widely administered, the Journal reports. FDA, for instance, said it plans to monitor reports of blood clots among people who received Johnson & Johnson's (J&J) single-dose vaccine after a large trial showed slightly more blood clots occurred among people inoculated with J&J's vaccine than those who received a placebo.

Some experts worry that countries' decisions to suspend the use of AstraZeneca's vaccine could make it more difficult to convince people to get vaccinated—even if it turns out that there's no link between the vaccine and the blood clotting.

Decisions to pause the use of AstraZeneca's vaccine "are bound to fuel hesitancy," said Stephen Griffin, associate medical professor at the University of Leeds, and could spur the spread of antivaccine views.

Likewise, Karl Lauterbach, a professor of epidemiology and a legislator in Germany's federal parliament, criticized Germany's decision to suspend the use of AstraZeneca's vaccine, saying it would have made more sense for the country to conduct an investigation into reports without suspending shots amid a surge in coronavirus cases in Europe. "In the third wave, which is now picking up speed, the first vaccinations with the AstraZeneca vaccine would be lifesavers," he wrote in a tweet (Strasburg/Pancevski, Wall Street Journal, 3/15; Lawler, Axios, 3/15; Herper, STAT News, 3/15; Falconer, Axios, 3/14; Grady/Robbins, New York Times, 3/15; Chiu, Washington Post, 3/15; Reuters, 3/15).