
New study estimates that 78% of Covid-19 cases are asymptomatic - guscost
https://www.bmj.com/content/369/bmj.m1375
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robocat
The discussion of this on the BMJ shows that this is poorly done (if it wasn’t
clear enough from some of the statements in the article). Best response was:

Yaneer Bar-Yam: [https://www.bmj.com/content/369/bmj.m1375/rapid-
responses](https://www.bmj.com/content/369/bmj.m1375/rapid-responses)

“”” Michael Day [1] claims there are 78% asymptomatic cases in China based on
the daily report of China National Health Commission on COVID-19 cases. The
report is based on an incorrect reading of Chinese reports [2], and both its
scientific and other inferences are incorrect. Given the importance of these
inferences, the article should be retracted immediately.

In particular, the article incorrectly assumes the report from China about 36
symptomatic and 130 asymptomatic are both from the same population. It also
assumes that the asymptomatic cases are not pre-symptomatic, i.e. they may
develop symptoms later, typically within a few days. Both these assumptions
are incorrect and lead to an incorrect understanding and conclusions.

For clarity, we report below the details of the Chinese report for March 31
through April 2. The newly reported results by China require careful
interpretation as we learn what each category represents.

For background, there are 3 distinct populations that the Chinese reports
refer to, A: International arrivals, B: Quarantined close contacts, C: The
general public. Understanding that there are more than one populations is
essential to understanding the Chinese reports.

CHINESE REPORTS

March 31 (reported one day later on April 1, this is the report Day[1] cites):
• 36 new symptomatic, 35 from A • 130 new asymptomatic from A, B, and C

Comment: If these were from the same population, then the percentage of
asymptomatic would be 130/(130+36)=78%. However this is not the case, as most
of the symptomatic individuals are from population A and most of the
asymptomatic are from population B and C (see subsequent days).

April 1: • 35 new symptomatic, all from A • 55 new asymptomatic, 17 from A • 9
asymptomatic cases convert on April 1 to symptomatic, all from A • 226
cumulative asymptomatic from A

Comment: We see that within population A, there are 35 new symptomatic and 17
new asymptomatic for this day. However, the statement that there are
conversions shows they may be pre-symptomatic rather than asymptomatic cases

April 2: • 31 new symptomatic, 29 from A • 60 new asymptomatic, 7 from A • 7
asymptomatic cases convert on April 2 to symptomatic, all from A • 221
cumulative asymptomatic from A Comment: As a result, within A, 29 are new
symptomatic, 7 are new asymptomatic

Comment: To understand the number of cumulative asymptomatic cases from A on
April 2 requires additional information from the Chinese report. The report
states that 101 asymptomatic cases, 5 of which from A, are released from
medical observation on the day of the report, after a 2-week quarantine and
consecutive negative test results. On April 2, asymptomatic cases from A start
at 226, there are added 7 new imported asymptomatic cases, subtracted 7
converted to symptomatic, and subtracted 5 released, leading to the 221
cumulative asymptomatic from A.

We note that the asymptomatic number from B and C is much larger than that
from A (849:226 as of April 1), but the 16 conversions are all from A. This
suggests that many of the current reported asymptomatic cases from
international arrival are pre-symptomatic. The high ratio between asymptomatic
cases from B and C compared to A makes sense due to the presence of long term
asymptomatic cases, that continue to test positively, remaining as a residual
from the large number of cases in China. “””

~~~
guscost
Thanks for posting this. Looks like we're going to have to wait for the
serosurveys before there's anything more reliable.

