Silent spread deadly

Diamond Princess – two words to make travel agents and virologists alike break out in a sweat. You might remember news footage of the Diamond Princess tied up in Yokohama, Japan, in early February, its 3711 passengers and crew under quarantine in a big

Covid-19 containment experiment.

The ship arrived after a 14-day cruise to China, Vietnam and Taiwan, during which time at least one passenger picked up the virus and it quickly spread onboard. But the extent of the problem wasn’t immediately clear as many of those infected didn’t show symptoms. They were silent carriers.

They also became the most dangerous people on the ship, some of them working in the kitchens, potentially serving up Covid-19 with mountains of buffet food. Under quarantine in Yokohama harbour, passengers and crew diligently checked temperatures and reported flu-like symptoms.

But the system failed. By the time extensive swab testing for coronavirus was under way on the Diamond Princess, it was too late: 712 people onboard were infected and 14 passengers died. That’s incredible when you think about it. Our official death toll stands at 22 – for the country.

Two new studies published in the past week serve to highlight the risks of asymptomatic infections. The silent cases could be what fuels a second wave of infections once lockdown restrictions ease off.

The first study, published in the journal Thorax looked at the spread of Covid-19 on another cruise ship, the Greg Mortimer, which set off from Argentina for an Antarctic cruise in mid-March with 217 passengers and crew.

Again, everyone onboard took their temperatures and made liberal use of hand sanitiser. Day eight saw the first case of fever. On day 20, when the ship was quarantined in Uruguay’s Montevideo harbour, 60 per cent, including a Kiwi couple, tested positive for Covid-19. But 81 per cent of those with the virus displayed no symptoms.

Another study published in JAMA Network Open looked at a group of 78 Covid-19 patients linked to infection clusters in Wuhan, and showed

42 per cent were symptomless, though the ‘‘asymptomatic’’ had milder damage to their immune systems.

These studies didn’t look at just how infectious asymptomatic carriers are. That’s a crucial remaining question to answer.

The silent cases could be what fuels a second wave of infections once lockdown restrictions ease off.