On 30 March, Canadian provinces suspended the use of the vaccine in people aged under 55. The pause was recommended by the country’s National Advisory Committee on Immunization (NACI), which pointed to reports of “rare cases of serious blood clots” in Europe. 1 These included instances of cerebral venous sinus thrombosis (CVST), the agency said. Most but not all cases had occurred in women aged under 55, and the syndrome typically appeared 4 to 16 days after receiving the vaccine. The fatality rate among those developing such clots had been as high as 40%, said Shelley Deeks, vice chair of NACI, announcing the recommendations.“There is substantial uncertainty about the benefit of providing AstraZeneca covid-19 vaccines to adults under 55, given the potential risks,” she said, presenting the recommendation as a temporary measure while more data are gathered.