In recent days, countries including Denmark, Ireland, and Thailand have temporarily suspended their use of AstraZeneca’s coronavirus vaccine after reports that some people who got a dose developed blood clots, even though there’s no evidence that the shot was responsible. The European Medicines Agency and the World Health Organization say the data available do not suggest the vaccine caused the clots and that people should continue to be immunized. Here’s a look at we know—and what we don’t.