BEIJING — Cutting off access to entire cities with millions of residents to stop a new virus outbreak is a step few countries other than China would consider, but it is made possible by the ruling Communist Party's extensive social controls and experience fighting the 2002-03 outbreak of SARS.CITIES AFFECTED: The first city closed off was Wuhan, with 11 million people, or 1 1/2 times the population of New York City.