Vaccines appear to be starting to curb new Covid-19 infections in the U.S., a breakthrough that could help people return to more normal activities as infection worries fade, public-health officials say. By Tuesday, 37.3% of U.S. adults were fully vaccinated against Covid-19, with about 2.7 million shots each day. Data from Johns Hopkins University shows the seven-day average for new U.S. cases has fallen below the 14-day average for more than a week, which epidemiologists said is a strong signal that cases are starting to slide again after a recent upswing.