WASHINGTON (AP) — A huge study of a single-dose COVID-19 vaccine is getting underway as top U.S. health officials seek to assure a skeptical public they can trust any shots the government ultimately approves. President Donald Trump is pushing for a fast decision. But Wednesday, the Food and Drug Administration’s chief pledged to a Senate committee that the decision will be based on science, not politics. Hopes are high that answers about at least one of several candidates could come by year’s end. The one-dose contender from Johnson & Johnson will be tested in 60,000 people in the U.S. and several other countries.