Five sailors on USS Theodore Roosevelt, which is sidelined in Guam due to the ongoing crisis, have tested positive for COVID-19 for the second time, according to the Navy.

The sailors have been taken off the ship. All five of them had previously tested positive and had already gone through at least two weeks of isolation.

Before rejoining the ship, they all had to test negative twice in a row with the tests being conducted a day or two before going back to the ship.

The Roosevelt, which is the fourth Nimitz-class nuclear-powered aircraft carrier in the United States Navy, has been at a port in Guam since late March after the outbreak was announced.

Since then, more than 4,000 of the 4,800 crew members have gone ashore for isolation or quarantine. Earlier this month, hundreds of sailors started returning to the ship to get ready to set sail again.

The Navy said on Friday that while onboard, the five sailors self-monitored and adhered to strict social distancing protocols.

“These five Sailors developed influenza-like illness symptoms and did the right thing reporting to medical for evaluation,” the Navy said.

The sailors were immediately removed from the ship and put back in isolation. Some sailors who came in contact with the five sailors were also removed from the ship.

Chief Pentagon Spokesperson for the Department of Defense Jonathan Hoffman said the coronavirus outbreak has been a learning process. “This is a very stubborn, infectious disease,” he said.

He explained that the medical crew was notified right away to determine who might have also been exposed and removed them from the ship.

As of May 14, more than 2,900 sailors have rejoined the ship and around 25 percent of the more than 1,000, who were tested positive, have now recovered, the Navy said.

The sailors were tested positive for COVID-19 using the nasal swab. In a few cases, the infection can be at such a low level that the virus goes undetected. It is unclear whether cases like these are actual relapses. It is also unclear whether people tested negative without being completely virus-free.