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COVID-19: asymptomatic cases on a cruise ship counted 81% of all those with positive tests by RT-PCR, and over half of ship was infected by one index case. Some of tests may have been false negative.


photo by Juraj Varga courtesy of

Thorax from the British Medical Journal published a brief communication that has been picked up by the general news media.  This communication describes the findings from a cruise ship to the Antarctic that departed in mid-March for what was to be a 21 day cruise. This cruise was to follow the route of the explorer Shackleton, who survived a years-long ordeal with his crew after wrecking on an exploration of the Antarctic in 1914-17.

Prior to departure, the 95-member crew and 128 passengers were all screened for symptoms of COVID-19.  No-one who had transited through China, Macau, Hong Kong, Taiwan, Japan, South Korea, or Iran in the previous three weeks was allowed to board.

Additional precautions prior to departure included “hand hygiene stations” all over the ship.  On the third day, the cruise was shortened because of travel restrictions that had been announced, and plans were made to return on day 14 to Ushuaia, Argentina.  On the eighth day, one passenger developed a fever.

Isolation precautions were started, with masks for all and all passengers confined to their cabins.  The crew wore N95 masks for contact with the passengers.  Meal service to the cabins three times a day was continued but the rooms were not “serviced.”  On the tenth day, three crew members had fever; on the next day, two passengers and crew had fever, and the day after that, three passengers developed fevers.

The ship sailed to Montevideo, Uruguay and arrived on the evening of the thirteenth day.  Most of the patients with fever had improved by that time.  Rapid testing kits for SARS-COV-2 antibodies were delivered and performed on six people who had fevers before, but came back negative on the fourteenth day.

Uruguayan officials refused permission to dock until nasal swab testing for COVID-19 by RT-PCR had been done.  Three additional people developed fever that day with mild cough and lethargy.  One passenger, a 68-y/o man, got worse and was evacuated to a hospital in Montevideo on the seventeenth day– he was intubated and tested positive by RT-PCR.  A total of eight people were evacuated due to “impending respiratory failure.”

The first evacuation came on day 17, and the others happened on days 20, 21, 22, and 24.  The seventh patient developed fever on day 23 and was evacuated on the 24th day.  One of the two ship physicians was the final patient to be evacuated, on the 27th day.  All of the evacuees tested positive.

All of the people on board were tested on day 20, the third of April.  Out of 217 still on board, 128 people tested positive by RT-PCR, including the six who had negative antibody tests.  In ten cases, two passengers sharing a cabin had discordant test results: one negative, the other positive.

Only 16 of the 128 who tested positive had any symptoms, all with fever and other “mild symptoms.”  Another eight were evacuated, four of whom were intubated.  One passed away.  A total of 24 patients who were positive had symptoms, representing 19% of all positive results.  104 with positive results represented 81% of all patients being asymptomatic.

The arithmetic stated in the report doesn’t completely add up;  there had been a total of 225 people on board, eight of whom were evacuated, leaving 217 to be tested.  Only two had been evacuated by the date given for testing, the 20th.

But 16 of the 128 on board who tested positive, plus the eight who were evacuated, adds up to 24 with symptoms (as stated)– but eight plus 128 should be 136 total positives… perhaps it was only 120 positives of the 217 on board, plus eight positive evacuees.  This remains to be clarified.

For 28 days, there had been no outside human contact with anyone on the ship (not counting the evacuations.)  The Uruguayan government allowed 112 ANZAC (Australia and New Zealand) people to be repatriated on the 28th day, and all the rest on the 32nd day.

In this unusual and isolated situation, 81% of people who were infected by SARS-COV-2 were asymptomatic.  Some of the RT-PCR tests may have been either false negative or false positive because ten cabins contained two passengers each who had mismatched test results.  The rapid antibody test proved unreliable in the acute phase of illness, as all tests performed were negative less than six days after the earliest patient developed symptoms.

Some passengers developed symptoms late, especially because the first one occurred eight days after departure and the last were on the 24th day; this suggests that “there may have been cross contamination after cabin isolation.”

This remarkable report indicates that the number of asymptomatic infections with COVID-19 is much higher than has been previously suggested (estimates had ranged from 20 to 50%).  Further evaluation of the routes of infection on this ship is a good idea; whether they occurred by contact, droplet, or aerosol transmission is unknown and could be puzzled out by interviews with the involved patients.

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