Three scientists who participated in an Antarctic cruise in March 2020, tell their story in a study, while the boat is struck by an epidemic of Covid-19. A story rich in information on how the epidemic is spreading and on the proportion of asymptomatic patients which would amount to 81%.
Jeffery Peter Green and his two colleagues did not go far to research the subject of their study. This doctor from the Royal Australian College in Melbourne embarked in mid-March 2020 aboard a cruise liner for a 21-day expedition to the Antarctic Peninsula, in the footsteps of the British explorer Ernest Shackleton in 1915-1917. This little pleasure trip will quickly end when an epidemic of Covid-19 breaks out on the ship. But it will also give the opportunity to our three scientists to study precisely the transmission of the virus on the boat. The results have just been published in the medical journal Thorax, and shed new light on the silent spread of the virus by asymptomatic carriers: 81% of passengers were tested positive without developing any symptoms.
Drastic hygiene precautions
So in mid-March, Jeffery Green, Alvin Ing and Christine Cocks embark on the boat in Ushuaia, Argentina, along with 125 other passengers and 95 crew members. At the time, the WHO already described the Covid-19 as a pandemic and all passengers are therefore subject to screening for the symptoms of the disease and to taking a temperature measurement before departure. In addition, no passenger having transited through one of the countries affected by the coronavirus during the previous three weeks is authorized to board. Numerous hand hygiene stations were installed throughout the ship and in particular in the dining room. Precautions which unfortunately will not prove to be sufficient.
The beginning of trouble
During the first seven days, everything is going well. The ship follows the planned route, passing Drake Passage, then Danco Island, Paradise Bay, Lemaire Passage and Deception Island. On the 8th day, a passenger declared a fever. He was immediately placed in isolation and all the passengers were then confined to their cabin. Staff distribute surgical masks and provide meals to passengers three times a day. Again, these precautions will not be enough. On the 10th day, three crew members showed signs of feverishness. On the 11th day, it is the turn of two passengers and another member of the crew, and three more people are affected on the 12th day.
Four out of five asymptomatic patients
VivaDiag serological tests are distributed on board, but all are negative on patients with fever. Distrustful, the Uruguayan authorities refuse to disembark passengers without an official PCR test. Eight passengers are still medically evacuated to hospitals in Montevideo, including a 68-year-old non-smoking man in serious condition. All evacuated patients will test positive for Covid-19. On the 20th day, the Uruguayan ministry finally delivers the famous PCR tests on board. Of the 217 passengers and crew members, 128 tested positive, or 59%. “In total, 19% of patients (24) tested positive had developed symptoms, the majority being therefore asymptomatic (104 patients, or 81%),” note the study authors. A sanitary corridor will finally be set up for the repatriation of passengers to their country of origin.
Jeffery Green and his colleagues bring several conclusions to their experience:
the prevalence of Covid-19 on cruise ships is probably very underestimated, which implies carefully monitoring passengers on their return to prevent community transmission;
rapid serological tests are not reliable in the acute phase;
the majority of virus carriers are asymptomatic;
the chronology of symptoms suggests cross contamination even after isolation in the cabin.
The Diamond Princess precedent
The case of the Diamond Princess, a cruise liner of 3,700 passengers and crew members, which left in February, had already been the subject of an in-depth epidemiological study. More than 700 people on board had been infected, constituting at the time the largest focus of contamination outside of China. Different studies had then been able to estimate the number of asymtomatic cases, the mortality of the virus or the rate of transmission. One of them, published on the Eurosurveillance site, had then calculated an asymtomatic patient rate of 17.9%. A low rate compared to that of the new study which could be explained in particular by the fact that the passengers were older than the average, and therefore more likely to fall ill.