WASHINGTON (Reuters) – A U.S. Navy investigation into the spread of the coronavirus aboard the Theodore Roosevelt aircraft carrier has found that about 60 percent of the roughly 400 sailors tested had antibodies for the virus, three U.S. officials told Reuters on Monday.

All 4,800 sailors on the Roosevelt aircraft carrier were tested for the coronavirus previously, and about a quarter tested positive. But in April the Navy and the Centers for Disease Control and Prevention (CDC) started conducting a serology test to look for the presence of specific antibodies.

If sailors who did not test positive show antibodies, that could suggest a higher rate of infection than previously known. Similar tests in Italy and elsewhere have indicated the presence of antibodies in people who did not test positive previously, giving a more accurate sense of the spread of the virus.

However, the serology test could also show that people who tested positive for coronavirus do not carry antibodies later, potentially raising questions about their immunity to the virus.

The spread of the virus on the ship put into motion a series of events that led to the firing of the ship's captain after the leak of a letter he wrote calling on the Navy to establish stronger measures to protect the crew.

One sailor from the ship died from the coronavirus and several others were hospitalized. But broadly, sailors, who are generally healthier and younger, fared better than the general population and most showed no symptoms whatsoever.

The officials, speaking on the condition of anonymity, said that only about 400 volunteers participated in the serology test, lower than the 1,000 volunteers that were sought.

They added a formal announcement was expected as early as Tuesday.

While the results could indicate a far higher presence of the coronavirus, one of the Navy officials said that may not be the case because of the way the study was carried out.

“The outbreak investigation did not encompass the entire crew, and the results of this study cannot be generalized to the entire crew,” the official said.

The serology test results follow data from the Roosevelt in early April, which showed that 60 percent of the sailors who were testing positive for the virus itself – not antibodies – were in fact symptom-free.

Medical groups, such as the American Medical Association, have warned that serology tests can lead to false positives.

In addition to the serology tests, volunteers were also swabbed again for COVID-19, the respiratory disease caused by the virus, as well as asked to answer a short survey.