UNITED STATES OF AMERICA : Scientists and drugmakers are beginning efforts to overcome Americans’ safety and other concerns about Covid-19 vaccines, while US health authorities ready a campaign to encourage widespread uptake.As vaccine candidates advance closer to US authorization, health and industry officials want to make sure as many people as possible get vaccinated to reach the level, known as herd immunity, that would protect even people who aren’t immunized.Yet large percentages of Americans, including those at high risk of contracting the virus, are reluctant, skeptical or opposed to taking a vaccine, according to surveys and researchers.