The decision by AstraZeneca to temporarily halt a phase three Covid-19 vaccine trial, is an unfortunate but necessary safety precaution in the development process US infectious disease expert Dr. Anthony Fauci has said, explaining that the rest of the enrollment of individual volunteers will have to be put on hold until researchers are able to establish what exactly caused an adverse effect in some volunteers. Fauci told CBS it will be some time before a proper vaccine can be rolled out.