The news that most European governments suspended the use of the Oxford-AstraZeneca COVID-19 vaccine has health experts worried that it could hamper an already slow vaccine rollout on the continent and fuel vaccine skepticism.

France, Italy, Spain and Germany are among a dozen European Union countries that suspended the use of the vaccine over concerns about blood clots occurring in vaccinated people.

There have been relatively few cases of blood clots — a few dozen among the 17 million people who have been vaccinated in Europe and the United Kingdom — and there’s no definitive evidence so far of a direct link.

But European regulators now say they aren’t ruling out that possibility. In a verdict issued March 18, the European Medicines Agency affirmed that the AstraZeneca vaccine was safe and effective, but recommended that vaccine patients be warned of the small risk of rare types of blood clots and said it would instruct healthcare providers to monitor for specific symptoms and safety risks.

The agency’s safety committee concluded that “the vaccine’s proven efficacy in preventing hospitalization and death from COVID-19 outweighs the extremely small likelihood of developing” these types of clots, which still haven’t been definitively linked to the vaccine.

Scientists in Germany are taking a close look at incidences of a type of blood clot called cerebral venous sinus thrombosis that’s occurring more often than expected among vaccinated people, to see whether it represents a potentially dangerous side effect. Three vaccinated people with that type of thrombosis have died, according to a German regulatory authority. And a Norwegian medical team has said that the vaccine was the most likely cause of a strong immune response that caused blood clotting in three health workers, one of whom died.

The AstraZeneca vaccine is not in use in the U.S. The company hasn’t filed an application for emergency use here, and it’s still recruiting for its 30,000-person U.S. vaccine trial. One report noted that it might take until April for the company to submit its data to the Centers for Disease Control and Prevention and the Food and Drug Administration.

Officials in Europe stressed that the vaccine’s suspension was precautionary and temporary, pending the regulatory ruling. Even so, vaccine skeptics seized on the news to promote the idea on social media that the COVID-19 vaccines are dangerous and even lethal.

“Breaking News: Europe suspends the use of Oxford-AstraZeneca COVID-19 vaccine,” reads a Facebook post. “1st vaccine to be suspended after thousands had died from vaccination worldwide but the media is not talking about it.”

The post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

There is no evidence that vaccines have caused any deaths, let alone thousands. But health officials in some European countries did view the reports of blood clots as cause for a deeper investigation.

Here, we take a closer look at the concern over blood clots and how health experts are responding to it.

Understanding the data on blood clots

Blood clots are not uncommon and can result from certain surgeries or medications, but they can also lead to serious events, including heart attacks and strokes.

According to the European Medicines Agency, about 30 “thromboembolic events,” or blood clots, have been reported among 5 million vaccinated patients in Europe.

The incidence of blood clots is lower when compared against the entire vaccinated population. An AstraZeneca press release said that 15 cases of deep vein thrombosis — a type of blood clot that develops in the legs — and 22 cases of pulmonary embolism — in the lungs — have been reported among the more than 17 million people who have received the vaccine in the United Kingdom and the EU.

One indication of a causal relationship would be if the rate of blood clots in vaccinated patients was higher than that in the general population, said Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and member of the Food and Drug Administration’s vaccine advisory committee.

This doesn’t seem to be the case for the Oxford-AstraZeneca vaccine when data from all vaccinations are taken into account.

“Blood clots are common,” Offit told PolitiFact. “It’s not surprising when you start to vaccinate millions of people that there are going to be people who still have blood clots. The challenge then for people who are monitoring these vaccines for safety is to determine whether this is occurring at a rate greater than the background rate.”

We don’t have exact epidemiological data on the rate of blood clotting disorders in the general public. However, one rough estimate of patients in seven European countries found that clotting disorders occurred in about 300 out of every 100,000 people in 2017.

Out of 5 million people vaccinated, this would come out to about 289 per week, almost 10 times the number of total blood clots among vaccinated patients reported by the European Medicines Agency.

Similar calculations by David Spiegelhalter, chair of the Winton Centre for Risk and Evidence Communication at the University of Cambridge, and Ann Taylor, chief medical officer for AstraZeneca, also arrived at the same conclusion: The rate of reported blood clots among vaccinated patients was lower than the expected rate in the general population.

Rare type of blood clot in Germany, Norway causes concerns

The European Medicines Agency was paying special attention to a rare type of blood clot reported in Germany called cerebral venous sinus thrombosis, or CVST.  Seven cases of CVST have been reported out of 1.6 million doses of the vaccine administered in the country, which is higher than the number of CVST that would normally occur in the unvaccinated population. The Paul Ehrlich Institute, an agency that advises the German government on vaccines, found that only one case of CVST should have occurred given the timeframe and number of doses administered.

In addition, a medical team in Norway said they have reason to suspect that the AstraZeneca vaccine triggered an immune response that caused clotting in three health care workers, one of whom died. Pal Andre Holme, the lead doctor on the team, said that he couldn’t be sure that the vaccine caused the clotting but saw it as the most likely cause.

Emer Cooke, the EMA’s executive director, told a news conference March 16 that there was “no indication that vaccination has caused these conditions.” But the relatively high incidence of CVST is the most concerning element of the vaccine data to regulators and vaccine experts at this time.

According to the EMA, most of these rare blood clots occurred in women under age 55. However, it is difficult to estimate a background rate for these events because they are so rare.

Public health experts worry about long-term effect of suspensions

Officials stressed that the suspension is precautionary and temporary, including Germany’s health minister Jens Spahn and French President Emmanuel Macron.

European countries that suspended the AstraZeneca vaccine were still moving ahead with vaccines from Pfizer and Moderna, which use a different technology to activate the immune system against the coronavirus.

But some experts are worried that the halt in Europe could cast a permanent pall over the vaccination effort and discourage people from getting their shots.

Many public health experts condemned the decision to suspend the AstraZeneca vaccine. Several countries, including Australia, Britain and Thailand, were moving ahead with it, saying that their data doesn’t show any health risks.

Offit said that he wished the European countries had pooled and carefully analyzed all of their data before halting vaccinations. “It’s very easy to scare people and very difficult to unscare them,” he said.

This article was originally published by PolitiFact, which is part of the Poynter Institute. It is republished here with permission. See the sources for these fact checks here and more of their fact-checks here.