United States" unless prepared to remain abroad indefinitely. On March 19, the State Department suspended routine visa services at all American embassies and consulates worldwide. By March 20, the U.S. began barring entry to foreign nationals who had been in 28 European countries within the past 14 days. American citizens, permanent residents, and their immediate families returning from abroad could re-enter the United States under the new restrictions, but those returning from one of the specified countries must undergo health screenings and submit to quarantines and monitoring for up to 14 days. In addition to the earlier travel restrictions in place, Trump extended this quarantine and monitoring requirement to those coming from Iran and the entirety of China. Flights from all restricted countries are required to land at one of 13 airports where the U.S. Department of Homeland Security (DHS) has "enhanced" entry screenings. At least 241 foreigners (including several Canadians), who had recently traveled in China and Iran, were denied entry to the United States between February2 and March 3. On April 21, President Trump announced a forthcoming executive order barring people from seeking green cards for a period of 60 days. Containment efforts within the U.S. On January 30, the WHO warned that "all countries should be prepared for containment, including active surveillance, early detection, isolation and case management, contact tracing and prevention of onward spread" of the virus. February 25 was the first day the CDC told the American public to prepare for an outbreak. By February, the CDC was exploring options to control the spread of COVID-19 in the United States. Six cities believed to be high-risk were selected for early "sentinel surveillance" to try to detect the virus in patients who did not meet CDC guidelines for testing; those cities were Chicago, New York, San Francisco, Los Angeles, Seattle and Honolulu. Very few tests were successfully completed within a five-week window. Once testing showed the disease was spreading among those without travel-related risk factors, public officials in California began to issue "stay at home" orders; it would be at least a week before similar orders were issued in other parts of the country. At a White House press briefing on April 1, Dr. Anthony S. Fauci said that, even though he expected social distancing rules can eventually be relaxed even before the availability of a vaccine, a vaccine will still be necessary to end the