Germany and France both temporarily suspended the use of AstraZeneca’s Covid-19 vaccine on Monday, joining a growing list of nations that paused use of the vaccine in recent days over concerns that it might be tied to blood clots.

Leading public health agencies, including the WHO, say that millions of people have received the vaccine without experiencing blood clotting issues, and they caution that experts have not found a causative link between the vaccine and the conditions. The company has also defended the vaccine as safe, amid the flurry of suspensions.

The safety scare is a setback for AstraZeneca’s vaccine, which has already struggled with a perception that it is a less desirable shot because it had a lower overall efficacy rate in clinical trials than some others. There is, however, extensive data showing that the vaccine is safe and effective, and especially good at preventing severe illness and death.

Public health experts expect medical conditions to turn up by chance in some people after they get any vaccine. In the vast majority of cases such illnesses have nothing to do with the shots.

Scientists also worry that suspensions could feed vaccine hesitancy at a time when some European countries are entering a third wave of the virus, and the world is in a race to inoculate as many people as possible, as dangerous virus variants proliferate.

In France, President Emmanuel Macron said on Monday that the country would stop using the vaccine pending an assessment by the European Medicines Agency on its use.

“The decision that was taken, in accordance with our European policy, is to suspend AstraZeneca vaccinations as a precaution, with the hope of quickly picking them up again if the European Medicines Agency permits it,” Macron said at a news conference. “We have a simple guide, to follow the science and the competent health authorities, and to do so in the framework of a European strategy,” he added.