AstraZeneca’s Covid-19 vaccine trial remains on hold in the United States pending a US investigation into a serious side effect in Britain even as other trials of the vaccine resume, according to sources familiar with the details.AstraZeneca on Saturday said it had restarted its trial in Britain after regulators completed their review of a serious side effect in one trial participant there.This was the first indication that the US trial will remain on hold until the US Food and Drug…