evidence: COVID vaccines and blood clots: five key questions [...] It has been a difficult week for two COVID-19 vaccines. On 13 April, US regulators urged health-care providers to temporarily stop using a vaccine made by Johnson & Johnson (J&J) of New Brunswick, New Jersey, because of six suspected cases of unusual blood clotting among nearly seven million vaccine recipients. [...] How could a COVID vaccine cause blood clots? Scientists race to investigate