Following the announcement of a possible link between the AstraZeneca vaccine and rare blood clots, the European Medicines Agency (EMA) is now investigating whether the vaccine is linked to a second very rare blood disorder: systemic capillary leak syndrome (SCLS). To date, five cases of SCLS in recently vaccinated people have been reported to the EMA. The regulator’s Pharmacovigilance Risk Assessment Committee is now working to see if a causal link with the AstraZeneca vaccine can be established – as it did following reports of blood clots – and whether new safety guidelines for the vaccine are needed. This investigation is a response to what the EMA calls a “safety signal”. These are unusual events that have been flagged up as possible side-effects of a given medicine, but which need researching in-depth for this to be confirmed.