The bicycle and pedestrian bridge known as Byggebroen in Copenhagen, Denmark on June 13, 2021. Denmark, a country of six million, has universal health care, so Danes can get treated more easily for diseases than Americans and often seek medical help earlier. (Charlotte de la Fuente/The New York Times) By AMELIA NIERENBERG and MAYA TEKELI The United States, a nation of 343 million people with a complex and overburdened health care system, is poised to adopt the childhood vaccine...