Post #551: World Health Organization report

Posted on March 16, 2020

This post summarizes some findings from the World Health Organization study of the Chinese coronavirus epidemic.  Here in the US, we often don’t look to international agencies for information.  So I thought this might be worthwhile.

The World Health Organization report.

You can reference any number of standard websites to find (e.g.) symptoms and such for this disease.  Here, I’m going to go to the World Health Organization (WHO) coronavirus website, as we here in the US typically don’t get our information from the WHO.  And then pull out any interesting information from their report on the Chinese coronavirus epidemic (available on this WHO web page as a .pdf).

So this assumes you know the basics, or you’ve read the last few posts I’ve written up.   Here are the things that surprised me:

Published mortality rates may significantly understate ultimate mortality rate.  The first huge surprise is that what has been circulated about the Chinese experience is a “crude mortality rate”.  That’s literally a count of those who have died, divided by all known cases.  Presumably, the same is true of the mortality rate published for Italy.

The problem is that there’s a long lag time between those two figures.  What you’ve got in the numerator of the crude mortality rate is “deaths, so far”, not “all deaths”.  From the WHO report, page page 14: “Among patients who have died, the time from symptom onset to outcome ranges from 2-8 weeks”.   So we know the count of diagnosed cases in real time.  But we won’t be able to count all the deaths among those cases for (in some cases) months yet.

Given the rapid growth curves for counts of cases (see just-prior post, currently the doubling time for coronavirus cases is on order of three days in most Western nations), and the long average lag between diagnosis and death for those who ultimately died, we are not yet counting many of the deaths that will ultimately occur.

So you have to look at the picture below and ask yourself, did the Chinese get vastly better at treating this, over time, or are they just missing most of the (ultimate) deaths on the right-hand side of the chart.  That is, does the crude mortality rate vastly undercount the ultimate mortality rate at the end of this graph, owing to the long lag between diagnosis and mortality?

Source:  WHO report, page 13.

In other words, is that a real decline in mortality (from 25% to under 1%! In less than two months!), or is that an artifact of the data?  Because this is the information being used to estimate an average 4% (crude) mortality rate.

To me, having worked with data of this type all of my life, that looks like it’s mostly an artifact of the data.  Particularly because there’s no known effective treatment for this.  I can’t say for sure without knowing the actual distribution of time to death, but looking at that chart, I’d guess that most of the (ultimate) deaths are actually missing from the right-hand side of it.  They haven’t occurred yet, and so are not counted.

Based on this, for sure, the published 4% crude mortality rate in China has to be taken as a lower bound for the actual (ultimate) mortality rate there.  Plainly put, a lot of the deaths haven’t happened yet and so haven’t been counted yet.  My guess is that the ultimate mortality rate from this may end up being substantially higher than the currently-reported 4% from the Chinese epidemic or 5% from the Italian epidemic.  And the reason for that is that it takes some time to die, so the one-point-in-time counts factor in all the cases, but only a part of the deaths.

Apparently no or few Typhoid Marys (asymptomatic cases spreading disease) “Asymptomatic infection has been reported, but the majority of the relatively rare cases who are asymptomatic on the date of identification/report went on to develop disease. The proportion of truly asymptomatic infections is unclear but appears to be relatively rare and does not appear to be a major driver of transmission.”  WHO report, page 12.

I need to be careful here:  It’s almost unheard-of for people to be spreading the disease without eventually showing the symptoms.  Doesn’t mean you can’t spread it during the incubation period prior to developing symptoms.  And there’s some risk-of-misinterpretation there, because maybe they only test people with symptoms.  But because of the “driver of transmission” statement, this appears to be a real finding (and not merely an artifact of testing strategy).

Children are not significant disease vectors.  Which in my experience differentiates this from every other virus on the planet.  This is from the WHO report, page 11, and I think is worth quoting in full (emphasis mine):

“Data on individuals aged 18 years old and under suggest that there is a relatively low attack rate in this age group (2.4% of all reported cases). Within Wuhan, among testing of ILI samples, no children were positive in November and December of 2019 and in the first two weeks of January 2020. From available data, and in the absence of results from serologic studies, it is not possible to determine the extent of infection among children, what role children play in transmission, whether children are less susceptible or if they present
differently clinically (i.e. generally milder presentations). The Joint Mission learned that infected children have largely been identified through contact tracing in households of adults. Of note, people interviewed by the Joint Mission Team could not recall episodes in which transmission occurred from a child to an adult.”

Mean incubation period is typically just under one week.  “People with COVID-19 generally develop signs and symptoms, including mild respiratory symptoms and fever, on an average of 5-6 days after infection (mean incubation period 5-6 days, range 1-14 days).

Most cases are mild but recovery times are long.

Most people infected with COVID-19 virus have mild disease and recover. Approximately 80% of laboratory confirmed patients have had mild to moderate disease, which includes non-pneumonia and pneumonia cases, 13.8% have severe disease (dyspnea, respiratory frequency ≥30/minute, blood oxygen saturation ≤93%, PaO2/FiO2 ratio <300, and/or lung infiltrates >50% of the lung field within 24-48 hours) and 6.1% are critical (respiratory failure, septic shock, and/or multiple organ dysfunction/failure).

“Using available preliminary data, the median time from onset to clinical recovery for mild cases is approximately 2 weeks and is 3-6 weeks for patients with severe or critical disease. Preliminary data suggests that the time period from onset to the development of severe disease, including hypoxia, is 1 week. Among patients who have died, the time from symptom onset to outcome ranges from 2-8 weeks.