Public-health officials in the U.S. are striving to keep tabs on thousands of Americans who have quarantined themselves at home after returning from mainland China to curtail the new coronavirus, adding to an epidemic response that is straining already-stretched local departments.

More than 5,400 people had been asked to self-quarantine in California alone as of Feb. 14, according to the California Department of Public Health. Hundreds more are self-quarantining in Georgia, Washington state, Illinois, New York and other states.

Public-health officials are following directions from the Centers for Disease Control and Prevention and the State Department to supervise these people, but many steps are being developed on the fly. States are using a variety of techniques to track people from calling in volunteers to make phone calls, to sending text messages and using electronic registries.

These people are separate from the Americans who are under stricter federal quarantine, including those housed at four U.S. air bases and the 328 who were recently evacuated from the Diamond Princess cruise ship. Those groups arrived from locations where the virus was rapidly spreading, whereas the people self-monitoring at home are thought to be at lower risk of having been exposed to it.

So far, there have been 15 confirmed Covid-19 cases in the U.S., not including the potentially positive cases among those evacuated from the Diamond Princess. While health officials have investigated 479 people for the virus as of Wednesday, 412 tested negative, with 52 cases pending, according to the CDC. The immediate risk to the general public is still low, health officials say.