Maria, the nurse in London, who feared that publishing her last name could jeopardize her green card, was turned away by American officials a second time when she tried to board a plane early this month, even though she said that she would be treating Covid-19 patients.

A few states that are facing major outbreaks have issued executive orders that allow some foreign health care workers who are already in the state to see patients. New Jersey, for example, is granting temporary licenses to doctors residing there who are licensed and in good standing in foreign countries. In New York, graduates of foreign medical schools are being permitted to treat patients after completing one year of residency, instead of the usual three.

However, the federal government has not broadly opened the way for foreign-trained workers to begin working.

Even before the coronavirus pandemic, demand for registered nurses in the United States was projected to grow from 2.9 to 3.4 million between 2016 and 2026, according to the Bureau of Labor Statistics.

The bureau had said an additional 203,700 nurses would be needed each year to fill new positions and replace retiring workers. However, only about 170,000 new nurses were licensed in the United States in 2018, the latest year for which data is available.

There is no guest-worker or nonimmigrant visa category for nurses. As a result, they must come to the United States with approved legal permanent residency, or a green card, a process that can take years to complete.

Meanwhile, hospitals in New York, New Orleans and elsewhere have been overwhelmed with patients, a backlog that sometimes forces people to wait hours to be admitted or to be sent home, potentially with life-threatening consequences.