In an ECDC report from Jan 17, it is said that

In China, 763 close contacts have been identified and monitored. Of these, 644 have completed the observation period, while 119 remain under medical observation. So far, none has tested positive for 2019-nCoV [7,10].

This seems a (very) striking result. With further time (and hopefully investigations) in hindsight, is there anything that explains why all those early tests of close contacts (of known cases) were themselves negative? Were the tests faulty in some way?

According to some much more recent media reports, Chinese tests kits sent to Europe or even Turkey have been found faulty, although the percentage is not made clear in most cases, except [oddly] for Turkey:

In Turkey, which imported its first batch of “several thousand” kits in early March, officials determined an accuracy rate of less than 35%, according to a senior official with direct knowledge of the matter. Their use was immediately suspended and new tests sourced from a different Chinese supplier. They arrived last week and had an accuracy rate of about 90%, according to the Turkish official.

(The article date is 1 April, but I don't think it's a hoax.) I don't know if this can possibly be related to the earlier Chinese non-detections or not... but also interestingly that low (~30% accuracy number) has been mentioned before:

In February, Wang Chen, a director at the state-run Chinese Academy of Medical Sciences, estimated that the nucleic acid tests used in China were accurate at identifying positive cases of the coronavirus only 30%-50% of the time.

  • 1
    Interesting question! I find it difficult to answer because we only have preliminary data, but so far the transmission rate varies drastically, some studies say household transmission rate is 83%, in a podcast Prof. Christian Drosten talked about unpublished data where the household transmission rate appears to be below 50%, early Chinese studies found it at 5%. I can not tell you where these differences come from. Regarding personal experience: All people I know personally who have been tested SARS-CoV-2 positive have not infected their
    – Narusan
    Commented Apr 9, 2020 at 19:10
  • [cont‘d] household members (they were all tested in Germany with the PCR protocol published by the Charité and all negative). I can only provide n=3 for the infected and n=10 for household members; and even so anecdotal evidence should hardly count. Nonetheless, I think we might be lucky and an 86% transmission rate in households is an overestimation.
    – Narusan
    Commented Apr 9, 2020 at 19:12
  • A bit of an aside: CGTN has gone to some unusual length to defend the exported Chinese kits; they say the users were using them wrong. covid-19.chinadaily.com.cn/a/202003/28/… Commented Apr 16, 2020 at 19:34

1 Answer 1


The first case of covid-19 was thought to have been diagnosed on the 17th November 2019 involving a 55 year old resident of Wuhan with viral pneumonia. With a R0 of 2.5 they would have already infected at least 2.5 other persons but for simplicity's sake let's say that on 17th November, 2019 there were 2 cases including the person diagnosed.

At that time the epidemic doubling time was said to be 5.2 days, let's say 5 days for simplicity.

The epidemic doubling time (the time it takes for daily incidence to double) was 5.2 (4.6–6.1) days before Wuhan was quarantined and public health interventions implemented within Wuhan

As an independent test to this, we know that Dr Li Wenliang reported to his WeChat group of classmates on the 30th Dec, 2019 that a SARS like pneumonia was being diagnosed in Wuhan Central Hospital where he worked. At that time there would have been about 2^^8.6 or 288 cases. About 5% would have needed ICU care so that means that about 14 cases would have passed through various hospital ICUs at that time.

Li raised the alarm after he saw seven patients with SARS-like symptoms. Li reported the suspected outbreak to his colleagues in a closed group on the WeChat social media platform after learning that patients were being quarantined.

The ECDC report refers to cases as of 17th January 2020, which is 61 days after the 17th November 2019 or approximately 12 * 5 doubling periods. So, this means at the 17th January 2020, they should have had 2^12 cases, or 4096, but the report only identified 44 cases. This indicates that they failed to identify the majority of cases at that time, and suggests that their testing kit was inaccurate. Adding to the problem was at that time they didn't realise that asymptomatic cases could also transmit disease.

And it was only 6 days later, on the 23rd January 2020, that Wuhan was locked down with neighbouring cities in Hubei province following shortly.

By 22 January 2020, the novel coronavirus had spread to major cities and provinces in China, with 571 confirmed cases and 17 deaths reported. Confirmed cases were also reported in other regions and countries, including Hong Kong, Macau, Taiwan, Thailand, Japan, South Korea, and the United States.

The lack of accurate testing has also been a problem in the USA, and elsewhere.





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