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Is COVID-19 Asymptomatic Transmission a Thing?

There is a paper by Shiyi Cao et al titled “Post-lockdown SARS-CoV-2 nucleic acid screening in nearly ten million residents of Wuhan, China”, first published on November 20.

During the 19-day period between 14 May 2020 and 1 June 2020 inclusive, there were 10,652,513 people aged ≥6 years in Wuhan (94.1% of the population) who were eligible for the test. Of this number, 9,899,828 persons participated, which is a participation rate of 92.9%.

Of the 9,899,828 participants, 9,865,404 had no previous diagnosis of COVID-19, and 34,424 were recovered COVID-19 patients.

So it appears 0.348% of those eligible for the test were previously infected. This seems to be generally consistent with other parts of the world at that time.

Now, here’s the interesting parts…

The screening of the 9,865,404 participants without a history of COVID-19 found no newly confirmed COVID-19 cases, and identified 300 asymptomatic positive cases with a detection rate of 0.303 (95% CI 0.270–0.339)/10,000.

So there were only 300 asymptomatic positive cases out of the tested population of over 9.8 million.

Of the 300 asymptomatic positive cases, two cases came from one family and another two were from another family. There were no previously confirmed COVID-19 patients in these two families. A total of 1174 close contacts of the asymptomatic positive cases were traced, and they all tested negative for the COVID-19.

So, although there were 300 asymptomatic positive cases, none of the 1,174 close contacts identified tested positive.

For the sake of completeness, there were a small number of re-positive cases.

There were 34,424 previously recovered COVID-19 cases who participated in the screening. Of the 34,424 participants with a history of COVID-19, 107 tested positive again, giving a repositive rate of 0.310% (95% CI 0.423–0.574%).
All asymptomatic positive cases, repositive cases and their close contacts were isolated for at least 2 weeks until the results of nucleic acid testing were negative. None of detected positive cases or their close contacts became symptomatic or newly confirmed with COVID-19 during the isolation period.

If these statistics are remotely correct, then it seems, according to the tests conducted in Wuhan, asymptomatic transmission is (or at least was) practically non-existent.

Also keep in mind that the test is the PCR test where “a cycle threshold value (Ct-value) less than 37 was defined as a positive result”, amongst other conditions. Setting aside the arguments for and against the PCR test, 37 cycles is not as high as in some places but it is higher than the 35 cycles as recommended by some to be a practical maximum. In other words, this test which yielded 300 asymptomatic positive cases and no positive cases amongst their close contacts is arguably over-sensitive to begin with.

This is just one study and it is possible that it is wrong but why all the alarmist hype regarding asymptomatic transmission? Why is there no meaningful discussion in lamestream media? Oh wait, never mind.


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