Portman so far is the only senator to announce participation in a clinical trial for a Covid-19 vaccine. Public confidence in a coronavirus vaccine has been declining throughout the year, according to polls conducted before this month’s promising news about candidates from Moderna and Pfizer-BioNTech. Half of Americans last month said they would not get the coronavirus shot, down from two-thirds this summer, according to Gallup polling. Portman joined the 60,000-person Phase 3 trial of Johnson & Johnson’s single-dose vaccine, receiving his shot on Nov. 5 — two days after the election. The trial Portman is participating in was paused last month after a participant got seriously ill.