The Compost Bulletin

What’s frightening about the COVID-19 corona virus is the overwhelming amount of inconclusive data we have about it. We are inundated with media speculation and hyperventilation, Chinese disinformation, and data that changes daily thanks to the treatment and quarantine initiatives governments are trying around the world. The fact that the disease has different rates of contagion and mortality in different parts of the world makes things more confusing. We can understand why it exploded in China, a place of censored information, pollution, smoking, and primitive socialized medicine. But why was it so virulent in Italy?

Surely Italy’s socialized medicine is better than China’s. Or is the problem Italy’s recent Silk Road Project agreement with China, which resulted in more than 300,000 Chinese workers entering Italy, mostly in the north, along with constant traffic between the two countries.  In retrospect, this may have had disastrous consequences for Italy. 

Fortunately, there is one almost “pure” location for corona virus data that is just now being examined. That is Princess Cruise’s Diamond Princess cruise ship. This was the ship moored along California’s coast while the government tried to determine the best way to deal with a floating virus petri-dish. OK, here are my questions. We had a perfect petri-dish corona virus disease (COVID-19) experiment with the cruise ship “Diamond Princess”. The ship provided a small sample of people, confined in close proximity for an extended period of time with no intervention made (other than segregation) to impede the spread of COVID-19 among the population. A more perfect experiment could hardly be imagined.  A study of what occurred on the Diamond Princes, with the breathtaking title Estimating the infection and case fatality ratio for COVID-19 using age-adjusted data from the outbreak on the Diamond Princess cruise ship (PDF) has been completed.  The results are informative.

On January 20, 2020, an 80-year-old passenger from  Hong Kong, China, embarked on in Yokohama, Japan.  He disembarked again in Hong Kong on January 25.  On February 1, six days after leaving the ship, he visited a Hong Kong hospital, where he tested positive for COVID-19. The ship was due to depart Yokohama for its next cruise on February 4, but announced a delay the same day to allow Japanese authorities to screen and test passengers and crew still on board. On February 4, the authorities announced positive test results for SARS-CoV-2 for 10 people on board, the cancellation of the cruise. The the ship and its passengers were held in quarantine where they would remain for another 14 days.  There were 3,711 passengers and crew aboard at the time.  A preliminary report based on the first 184 cases by Japan’s National Institute of Infectious Diseases (NIID) estimated that most of the transmission on the ship had occurred before the quarantine.

For 15 days, before they knew it was a problem, the epidemic raged on the ship, with infected crew members cooking and cleaning for the guests, people all eating together, close living quarters, lots of social interaction, and a generally older population. That would seem like a perfect situation for an overwhelming majority of the passengers to become infected. Despite that, some 83% of the passengers never got the disease at all … why? What an excellent question. Why did 83% of the ship’s passengers walk away unscathed? It’s an especially good question because the majority of passengers were in the 60-79 age group, weighted slightly more heavily towards the 70-79 cohort. In other words, by the time the ship docked, it should have been a floating morgue, but somehow it wasn’t.

When those on the ship who did not get sick are examined, some very intriguing things are discovered.  The chart at left shows the age distribution of COVID-19 Infections on the Diamond Princess. (The statistics are from February 19.  As of March 5 the total number of cases has risen to 696, including some of the Japanese officials who came on board the ship after quarantine.  The percentages and distribution did not change appreciably though.)

There was not a lot of difference between young and old passengers in terms of how many didn’t become infected by corona virus. For example, 60-69 year-old passengers stayed healthier than teenagers. And three-quarters of the oldest group, those over 80, never get the virus.  The number of people who got sick though is the most interesting finding. Because the Diamond Princess was monitored, we know precisely how many people got sick and we know that they would never have been noticed were they not on the ship.

It is particularly valuable to know that about half the cases were asymptomatic. It lets us adjust the mortality rate calculated from observations, since half of the cases are symptom-free and likely unobserved and unreported. It also gives a better idea of how many cases there might be in a given population. 

The total corona virus deaths in a several countries are wildly different and do not resemble what happened on the Diamond Princess. Italy is the most serious, with 2,978 deaths and 35,713 confirmed cases, giving a mortality rate of 8.3%.  On the other end Germany with 13,749 confirmed cases has recorded only 42 deaths, for a .31% rate.  The Diamond Princess mortality rate, with a fully known population, was 7 cases out of 696 known infections.  The infection rate was 18.7%, typical of previous influenza pandemic infection rates.

The  charts below  show the percentage of passengers on the Diamond Princess by age group who did not contract corona virus.  Alongside that is a chart showing the percentage of passengers by age group who had contracted corona virus but were asymptomatic.  Interestingly the age groups >50 had a 55% probability of being asymptomatic, higher than lower age groups.  Again, a curious distribution. Young and old were more likely to be symptom-free, while people in their 20s, 30s, and 40s were more likely to show symptoms. Who knew?

There were a total of 7 deaths among those on board. All of them were in people over seventy. So even though the generally young were more likely to show symptoms if they had it, it hits old people the hardest. Finally, according to the study, the age-adjusted infection fatality rate was 1.2% (0.38%–2.7%). Note the wide uncertainty range, due to the small number of deaths. This is all good news. 83% of the people on the ship didn’t get infected, despite perfect conditions for transmission. If you get it, you have about a 50/50 chance of showing no symptoms at all. And the fatality rate is lower than the earlier estimates of 2% or above.

It is particularly valuable to know that about half the cases are asymptomatic. It lets us adjust a mortality rate calculated from observations, since half of the cases are symptom-free and likely unobserved. It also gives a better idea of how many cases there are in a given population.