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expert reaction to opinion and data analysis on whether expanding the UK covid-19 case definition would improve the UK’s pandemic response

In a data analysis and opinion piece published in the BMJ, experts suggest the UK should expand its official list of symptoms for defining COVID-19 to prevent cases being missed and help improve the UK’s pandemic response.


Dr Hamish Houston, Clinical Research Fellow, National Heart and Lung Institute, Imperial College London, said:

“Crozier et al make a strong argument which considers the available scientific evidence on the subject to date.

“The appropriate selection of symptoms within case definitions has a direct impact on transmission dynamics. Case definitions which include more symptoms will be more sensitive, but less specific. Specificity matters because testing capacity is a limited resource and may be overwhelmed if symptom definitions are made too broad.

“The UK public health response to COVID-19 has become heavily focussed on lateral flow testing which certainly have a role in the pandemic response. However, these tests should not be used as tests to rule out COVID-19 infection in individuals with symptoms given their low sensitivity and potential to miss infectious cases.

“As well as the roll-out of lateral flow testing, there have been huge increases in UK RT-PCR testing capacity since the start of the pandemic. We share the authors concerns that the current list of symptoms which prompt symptomatic testing using RT-PCR is too narrow to make best use of this resource. Data from large scale cross-sectional studies such as REACT and the Covid Symptom Study App would support the inclusion of more symptoms in the national case definition.

“Individuals are most infectious early in the course of infection. We have completed longitudinal prospective study of recently exposed SARS-CoV-2 contacts to understand how adding symptoms to the national case definition might help identify people with infection earlier in the course of infection. Our, as yet unpublished, findings agree with those of REACT and the Covid Symptom Study App that a broader case definition would allow more effective early case identification.”


Prof Jon Deeks, Professor of Biostatistics and head of the Biostatistics, Evidence Synthesis and Test Evaluation Research Group, University of Birmingham, said:

“Identifying, testing and isolating Covid-19 cases early, and tracing their contacts, is vital to containing the spread of the pandemic.  Our approach has involved only directly testing people with one of three cardinal symptoms (new continuous cough, fever and change of smell or taste).  Crozier and colleagues describe how this set of symptoms has not been updated despite multiple research studies reporting how sore throat, headache, runny nose, fatigue and chill like symptoms are often the first symptoms that occur, particularly in younger people who are our current reservoir of infection.

“Whilst broadening the case definition will increase demand on testing, it will be worthwhile if it leads to earlier detection and associated reduction of spread in enough individuals.  Several local Public Health groups (e.g. are already directing individuals to obtain PCR testing as a precaution if they present with these symptoms, and may have data to show how many new cases are detected who first present with symptoms outside of the core definition. 

“Lateral flow tests are not provided to test people who present with Covid-19 symptoms, but there are many reports on social media of individuals resorting to using them when they are unwell, but not with the key symptoms.  They are not well suited as they often fail to identify early stages of infection as they cannot detect low viral loads common at this time.  To reduce disease spread it continues to be important that all those at high risk of infection, which includes many with these symptoms, can access PCR testing as quickly as possible, which requires these changes to be made.”


Dr Alexander Edwards, Associate Professor in Biomedical Technology, Reading School of Pharmacy, University of Reading, said:

“This raises a valid point, which is exactly how do you spot cases in the community, given the wide range of possible symptoms.  The big difficulty is in accurately finding people who are infected, without needlessly testing other people with other symptoms.  Many of the symptoms are very common – and the best way to judge the likelihood of someone having COVID-19 is a combination of symptoms, their risk of exposure, plus testing.  Self-reporting symptoms is especially difficult because even the words we use can be interpreted differently.

“Spotting people who are infected is just part of the story, however, because unless you can effectively support infected individuals to isolate from others and thereby break the chain of infection, testing alone doesn’t reduce the burden of this disease.

“What this study does pick up on is a clear gap between asymptomatic testing with lateral flow tests, and symptomatic PCR testing – targeted at three particular symptoms.  There is no clear guidance provided with the asymptomatic rapid tests if they would work equally well if you have different symptoms – for example a runny nose or sore throat.

“We must also remember that lateral flow home tests are NOT able to rule out infection.  So if people are using these home tests when they are uncertain if they fit the NHS Covid-19 symptoms, as an alternative to booking PCR tests, that could also lead to missed cases or delayed diagnosis.”



‘Could expanding the covid-19 case definition improve the UK’s pandemic response?’ by Alex Crozier et al. was published in the BMJ at 00:01 UK time on Thursday 1 July 2021.

DOI: 10.1136/bmj.n1625



Declared interests

Prof Jon Deeks: “JD is an external advisor to the MHRA and WHO, and leads the Cochrane reviews of the accuracy of tests for SARS-CoV-2 supported by funding from the Foundation for new Innovative Diagnostics.”

Dr Alexander Edwards: “Nothing to declare.”

None others received.

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