Health Canada is expected to release updated data today as concerns about vaccine safety emerge again. Canada reported its first case of vaccine-induced blood clots linked to Oxford-AstraZeneca yesterday, and the United States put the brakes on the Johnson & Johnson vaccine following six reports of blood clots there. The Johnson vaccine held immense promise because its single-dose regimen and relatively simple storage requirements would make it easier to use, especially in less affluent countries. The clots, which happened six to 13 days after vaccination in veins that drain blood from the brain, occurred together with low platelets -- the fragments in blood that normally form clots.