But while the Navy and civic leaders in Guam struggled to quell the spread of the virus, naval officials and researchers at the Centers for Disease Control and Prevention began a medical investigation into the outbreak, the results of which were released on Tuesday.
The study found that, among a few hundred service members who volunteered to be tested and questioned about their experiences onboard and while in Guam, more than a third had enough functioning antibodies to the coronavirus to indicate they could have some protection against the virus, at least for a limited time.
Some were still showing the presence of neutralizing antibodies, which block the virus from binding to cells, three months after the onset of symptoms.
“This is a promising indicator of immunity,” said Daniel C. Payne, an epidemiologist and one of the lead authors of the study, which was undertaken in conjunction with the Navy. “We don’t know how long-lasting, for sure, but it is promising.”
The report noted that while many studies have focused on older adults and those with underlying conditions, this one was able to examine younger, generally healthy adults. Approximately 1,000 crew members were infected with the virus at the time of the study; a few were hospitalized and one died.
The investigation included 382 sailors, who were generally healthy young adults, both male and female; it confirmed what other studies have found, that some of the earliest symptoms included loss of smell and loss of taste. It also found that 18.5 percent of participants in the study were asymptomatic.
In late April, after the outbreak began, naval medical officers began testing the volunteers. Blood samples were sent overnight from Guam to Georgia, where they were tested at the C.D.C. labs, using their own antibody tests.
The first testing found the presence of coronavirus antibodies in 228 (59.7 percent) of the volunteers. Of those, 135 (59.2 percent) had antibodies powerful enough to neutralize the virus and thwart infection.
“This is still a new disease, and there is so much we do not know,” Dr. Payne said. “This is not conclusive, but we are encouraged by it.”
Dr. Payne noted that it could take weeks for neutralizing antibodies to develop after infection, and he guessed that some of the tests were performed on infected sailors before their bodies had a chance to ramp up production. If so, the percentage of patients with active, functioning antibodies could be higher.
The investigation discovered that for a dozen participants who were tested for antibodies more than 40 days after the onset of symptoms, eight still had neutralizing antibodies. That included two who were tested more than three months after symptoms first appeared.
“There were some people who continued to have robust neutralizing antibodies at longer time periods within our own study,” Dr. Payne said.