» US officials may need 2 weeks or more to determine if Johnson & Johnson's vaccine causes rare blood clots US officials may need 2 weeks or more to determine if Johnson & Johnson's vaccine causes rare blood clots Aria BendixApr 18, 2021, 00:03 IST A pharmacist at the Hartford Hospital in Hartford, Connecticut, fills a syringe with the Johnson & Johnson COVID-19 vaccine.Joseph Prezioso/Getty Images US agencies may need two weeks or more to know if Johnson & Johnson's shot causes rare blood clots. The But medical experts worry that pausing the shot for much longer could increase vaccine hesitancy. US federal agencies may need two weeks or more to know whether Johnson & Johnson's