The USNS hospital ships Mercy and Comfort have treated fewer than 20 patients combined since the ships arrived in Los Angeles and New York City, respectively, according to the commanding officers of the vessels.

That means the majority of the hospital ships’ vast spaces remain vacant — each can accommodate 1,000 hospital beds.

According to the Mercy’s commanding officer Capt. John Rotruck, the hospital ship has treated a total of 15 patients — five of which have been discharged already — since the Mercy arrived in Los Angeles on Friday. Meanwhile, the Comfort arrived in New York City on Monday and has since treated three patients, the Comfort’s commanding officer Capt. Patrick Amersbach said.

Neither the Comfort or the Mercy are accepting walk-in patients, the captains said. Rather, patients are referred to the ships from local hospitals, who are screening and conducting COVID-19 tests on patients before boarding the vessels to prevent the spread of the virus.

“It’s really driven by demand from the local hospitals based on their current capacity,” Rotruck told reporters Thursday.

“If that demand signal ramps up, we’ll certainly be ready to accommodate that,” Rotruck said.

Rotruck said the Mercy saw a variety of patients the past week, including those involved in traumatic accidents and those with gastrointestinal issues. Patients are not billed for the care they receive aboard the ship, he said.

However, the hospital ships are not providing obstetric or pediatric care, and are strictly seeing non-COVID-19 patients to allow local hospitals more capacity to treat patients suffering from the pandemic.

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In the event that the hospital ships were ordered to treat COVID-19 patients, both Rotruck and Amersbach said they would adjust accordingly. Neither were certain what that mission would look like, though they offered a few predictions.

“Essentially we would transition from a non-COVID-19 patient treatment facility to a COVID-19-only patient treatment facility,” Rotruck said. “So in other words, we would probably have to transfer the non-COVID infected patients off the ship and become 100 percent COVID operation.”

Likewise, Amersbach predicted that the beds would be reconfigured on the ship to provide more space.

The Mercy is equipped with 800 Navy medical personnel and support staff, while the Comfort has more than 1,000 onboard.

To date, there are more than 226,000 COVID-19 cases in the U.S. — including nearly 47,000 in New York City alone, according to Johns Hopkins University. In comparison, Los Angeles has just over 3,500 cases of the virus.

Cases have continued to rise among the military this week as well. According to the Pentagon, 893 service members have tested positive for COVID-19, along with 306 civilians, 256 dependents and 95 contractors.