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Of passengers and crew on a Southern Hemisphere ocean cruise who tested positive for COVID-19, only about 20% ever developed symptoms, researchers found.

The vessel carried 217 passengers and crew, with 128 ultimately testing positive for SARS-CoV-2. Of these, only 24 were recorded as showing symptoms, reported Alvin Ing, MD, of Macquarie University Faculty of Medicine and Health Sciences in Sydney, Australia, and two colleagues in a Thorax.

Moreover, 6% required medical evacuation, 3% were intubated and ventilated, and one person died, the authors noted.

Ing and colleagues detailed their experience of "the first instance of complete COVID-19 testing of passengers and crew on an isolated cruise ship." Ing himself and another author were passengers on the vessel; the third author was the expedition physician. The cruise was intended to follow part of the famous Antarctic journey of Ernest Shackleton and his men in 1915-1917, with stops at Elephant Island, where the explorers spent one winter, and South Georgia, to which Shackleton and five others sailed in an open boat.

The cruise carrying Ing and colleagues was in mid-March. Organizers took what, in retrospect, still appear to be thorough precautions. All 128 passengers and 95 crew were screened for symptoms, including temperature checks, before boarding. No passengers or crew had traveled through existing COVID-19 hotspots in mid-March, including China, South Korea, and Iran. Symptom checks were performed regularly after departure as well.

In addition, "multiple hand hygiene stations were positioned throughout the ship and especially in the dining areas," the authors said.

Argentina itself had 45 cases on March 15, though that had ballooned to 820 by March 31,

Once a passenger developed fever on day 8 -- by this time, the vessel had already been rerouted due to new border controls implemented to contain international COVID-19 spread -- isolation protocols were put into place. Passengers were confined to cabins and issued surgical masks. Full personal protective equipment was utilized for contact with febrile patients, and N95 masks were used by the crew for contact with passengers. Crew delivered meal services to passengers' doors, but rooms were not serviced.

On day 10, three crew members developed fever, two passengers and one crew member were febrile on day 11, and three passengers had a fever on day 12. Rapid COVID-19 testing kits were performed on six passengers and crew with initial fevers, and all tested negative on day 14. Three additional passengers developed fever on day 14.

There were eight medical evacuations, all with respiratory failure, including one of the ship's two physicians who suffered hypoxemia.

On day 20, passengers and crew were tested with reverse transcription polymerase chain reaction (RT-PCR) via nasal swab for SARS-CoV-2. The 128 individuals testing positive included some who tested negative via rapid antibody testing.

Sixteen of those with symptoms had only mild illness. Of the eight whose conditions required evacuation, four were intubated with mechanical ventilation; one of them died. (Ironically, all of Shackleton's group survived.)

Ing and colleagues noted that the tests used on passengers appeared unreliable -- both the rapid antibody tests as well as the PCR tests showed "significant" false-negative rates, they said.

Because the vast majority of infected passengers were asymptomatic, the authors concluded, "[t]he prevalence of COVID-19 on affected cruise ships is likely to be significantly underestimated, and strategies are needed to assess and monitor all passengers to prevent community transmission after disembarkation."

Cruise ship travel is scheduled to resume in the U.S., with the world's largest cruise ship company, Carnival, after plans to resume operations in August were announced, according to the New York Post. are reportedly scheduled to resume operations in June.

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