NOTTINGHAM, England — The number of people who are walking around while unknowingly contagious with COVID-19 may be far more than most assume, according to a new study. Researchers examined a cruise ship that set sail in March, and found that more than eight in 10 passengers or crew members who tested positive for coronavirus showed no symptoms at all.
In all, 128 passengers and 95 crew members were on board the ship, which departed from Argentina in mid-March for a planned three week journey through the Antarctic. At the time of departure, would-be passengers who had passed through countries with high COVID-19 infection rates at the time weren’t allowed to board, and everyone who was allowed on had their temperature taken. There was also hand sanitizer readily available all over the ship. So, at least some precautions were taken.
The first fever on the ship was recorded eight days after embarking. This development immediately sparked “infection control measures,” meaning all passengers had to stay in their cabins, daily service was stopped (besides meals), and all crew members were required to wear PPE when interacting with potentially sick passengers.
By this time, Argentina had closed its borders, so the ship instead sailed to Uruguay. Upon arrival, eight passengers / crew members had to be taken to local hospitals due to respiratory failure. By day 20 of the journey, all remaining people on the ship were officially tested for COVID-19. More than half (59%) tested positive. Among those coronavirus positive passengers, the vast majority (81%) did not show any symptoms.
Furthermore, some of the tests on board may have been inaccurate; 10 cabin mates didn’t share the same test results. This indicates at least a few false negative tests.
In summation, these findings depict a virus capable of spreading quickly and decisively within closed environments. While it was already known that COVID-19 can spread extensively in a setting like a cruise ship, the study’s authors say that current infection estimates on cruise ships are likely “significantly underestimated.” Consequently, they recommend that all cruise ship passengers moving forward be monitored after ending their trip.
Also, due to the seeming prevalence of false negative tests, two rounds of COVID-19 testing may be prudent for cruise ship passengers.
“It is difficult to find a reliable estimate of the number of COVID positive patients who have no symptoms,” comments Professor Alan Smyth, joint editor in chief of the British Medical Journal. “As countries progress out of lockdown, a high proportion of infected, but asymptomatic individuals may mean that a much higher percentage of the population than expected may have been infected with COVID.”
On a broader level, this study suggests that global data on COVID-19 infection rates may be woefully inaccurate and underestimated as well.
The study is published in Thorax.