Johnson & Johnson has temporarily halted its Covid-19 vaccine trial due to an unexplained illness in a participant. Johnson & Johnson had earlier this month started human trials of vaccine that may work as a single shot. The company has since begun dosing up to 60,000 volunteers. News of the trial halt comes after AstraZeneca temporarily stopped tests of its vaccine after a trial participant fell ill. That study has resumed in a number of countries but remains halted in the US.