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expert reaction to the latest change in case definitions in China for COVID-19

The Chinese authorities have changed their case classification for COVID-19, leading to a spike in case numbers.  


Dr Mike Tildesley, Associate Professor, University of Warwick, said:

“Essentially, it looks like there has been a change in what is defined as a “confirmed case”. Previously, cases were confirmed only after positive laboratory tests but now it looks like cases are being declared confirmed based upon clinical diagnosis of symptoms. These cases were previously defined as “probable” cases. Given this change in classification, it is unsurprising that we have seen an increase in confirmed cases, but this does not necessarily mean that we are seeing an increase in the rate of infection. What it does mean is that we may now have a more accurate measure of the true extent of the outbreak, which in the longer term will improve our ability to predict how the virus may spread in the future. As for the implications of this change in confirmation, we will have more information on this in the next few days, as we monitor whether the number of daily confirmed cases starts to rise or declines again.”


Prof Mark Harris, Professor of Virology, University of Leeds, said:

A paper published in the journal Radiology ( on Feb 12 may shed some light on the change in case definition and diagnosis.  The team examined 167 patients and identified 5 for whom the initial test for the presence of the virus in mouth swabs was negative.  These patients were examined by CT scan which revealed signs of pneumonia.  All 5 patients subsequently tested positive for virus in mouth swab.  It should be noted that these 5 patients had all presented with symptoms of SARS-CoV-2 infection (eg fever, cough).  This suggests that a small number of patients may show signs of lung infection prior to a positive identification of virus in mouth swabs. However, it would be useful to have some official confirmation from the Chinese authorities about how the procedures for diagnosis have changed to fully assess the implications.


Prof Paul Hunter, Professor in Medicine, University of East Anglia (UEA), said:

“This does not change anything about trajectory of the epidemic.  It is solely an administrative issue. I doubt the real number of new cases is much different from yesterday.

“Essentially they are accepting a clinical diagnosis which previously would have only qualified as a suspect case. This will effectively have moved most of the previously suspected cases into confirmed case.  Many of the suspect cases will ultimately be proven to be COVID19 (a confirmed case) others will actually be other illnesses altogether or remain lacking a laboratory diagnosis. Of those suspect cases lacking a laboratory diagnosis some but not all will be COVID19. So, in reality we are now including in the final figure cases that are almost certainly COVID19 but because of various factors lab tests were not done. On the other hand the figure will include cases of illness that were not caused by COVID19.

“The issue remains how are we going to be able to say what is happening with the trajectory of the outbreak when the cases definitions change mid-way through the epidemic? Will the figures be backdated? Also, what about cases that have a clinical diagnosis but negative lab tests, are they included in the confirmed cases or not?

“I have no problem with people using different case definitions but it would be great if they could be consistent,  or if they do change, run both in parallel for a few days so that no one believes the epidemic has suddenly got a lot worse. I suspect but can’t be certain that the underlying trend is still downwards.”


Prof Mark Fielder, Professor in Medical Microbiology, Kingston University, said:

“The spike in numbers largely reflects a change in reporting in that we are seeing a clinical diagnosis bought into the equation. This looks to be in place of the nucleic acid based tests used before. I suspect this is to gather more of the patients apparently missed in the data. I am reassured that despite the rise in numbers the death rate still stands at around 2%.

“As it stands, I think the data and the situation remain one that we need to have global eyes fixed upon, we should be concerned and take it seriously, but not worry. 

“Regarding the predication around when the outbreak will be over, by the Chinese epidemiologist who suggests that it will die down at the end of April, and other suggestions that it will carry on for a lot longer; the April deadline might be a little optimistic but we need to carry on what we are doing in terms of infection control and so on that means we limit wherever possible the spread and see what happens with this ever evolving situation. The seasons and their change might have an impact on the spread of the virus, in line with the comments made by Prof Chris Whitty this morning.”


Prof Paul Hunter, Professor in Medicine, University of East Anglia (UEA), said:

“When investigating any outbreak or epidemic we usually have several case definitions. The case definitions being used in the current epidemic are given below.

“Early in the outbreak WHO in its daily situation reports listed both Suspect and confirmed cases. Then for some time listed only confirmed cases until 12 February when it stated reporting suspect cases again. Today’s count seems to be roughly the sum of confirmed and suspect from yesterday. But I do not know whether they have changed the case definitions, started presenting confirmed and possible cases together as a single figure or have a new diagnostic method. I do not think the last is likely.

“If the figure now included suspect cases , many of the suspect cases will ultimately be proven to be covid19 (a confirmed case) others will actually be other illnesses altogether or remain lacking a laboratory diagnosis. Of those suspect cases lacking a laboratory diagnosis some but not all will be covid19.

“It is notable that WHO has not updated its dashboard with Chinese figures today suggesting to me that they have not yet decided how to deal with this.

“In determining whether the epidemic is declining from day to day or not we have to be careful only to compare like with like. Until we know more about the new method and how it compares to previous numbers we cannot use todays figures of how the epidemic is progressing. It almost certainly does not mean that there has been a resurgence of the epidemic overnight”


Additional Information from Prof Hunter

Suspect case

A: Patient with severe acute respiratory infection (fever, cough, and requiring admission to hospital), AND with no other etiology that fully explains the clinical presentation AND a history of travel to or residence in China during the 14 days prior to symptom onset,  


B: Patient with any acute respiratory illness AND at least one of the following during the 14 days prior to symptom onset:

  1. a) contact with a confirmed or probable case of 2019-nCoV infection, or
  2. b) worked in or attended a health care facility where patients with confirmed or probable 2019-nCoV acute respiratory disease patients were being treated.

Probable case

Probable case: A suspect case for whom testing for 2019-nCoV is inconclusive or is tested positive using a pan-coronavirus assay and without laboratory evidence of other respiratory pathogens.

Confirmed case

A person with laboratory confirmation of 2019-nCoV infection, irrespective of clinical signs and symptoms.



All our previous output on this subject can be seen at this weblink:  


The SMC also produced a Factsheet on COVID-19 which is available here:


Declared interests

None received. 

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