Even as China takes more stringent measures to limit the movement of the vast country’s population during the biggest travel period of the year, and as the United States and other countries move ahead with evacuation plans, there are increasing fears that a quarantine will not be enough to stop the spread of the new coronavirus, which so far has infected at least 4,428 people in China and killed at least 106. Here’s what we know:

The U.S. Centers for Disease Control and Prevention raised its travel warning Monday to a level 3, its highest alert level, urging U.S. citizens to avoid all nonessential travel to China.

● With at least 106 dead so far, Beijing has broadened the extraordinary quarantine to more than 50 million people, but the mayor of Wuhan, the outbreak’s epicenter, said 5 million people have already left his city.

● China’s health minister said the coronavirus is increasing in virulence and now could be contagious even before people exhibit symptoms, making apparently healthy people possible carriers.

● A scientific assessment of the spread of the disease, assuming an optimistic 90 percent quarantine, still predicted more than 59,000 infections and 1,500 deaths — twice the toll of the 2002-2003 SARS outbreak.

The CDC said on Monday there were no new confirmed cases of coronavirus overnight but that it is investigating 110 people in 26 states.

● In the United States, health officials confirmed five cases of the pneumonia-like illness, while infections also have been confirmed in France, South Korea, Japan, Nepal, Thailand, Cambodia, Singapore, Vietnam, Taiwan, Canada, Sri Lanka and Germany. We’re mapping the spread here.