In the news, especially across Europe, is the inability to meet recent growing demand for Covid-19 swab testing. This threatens return to normal life and impacts on the functioning of educational, health and care institutions. Testing authorities are firmly advising only those with symptoms should be tested, but does this make sense? In this post I present an epidemiological analysis to inform thoughts on this decision.
One problem is the low pick-up rate
- In the graph below, I provide the percentage of those who had a test who were positive in the latest full week
- The UK (depending on how one interprets the results) does badly with less than 1% of those tested being positive
- The UK government questions the value of such a high proportion with a negative test result
- We do not know how many of those tested in each country had symptoms, or indeed what symptoms they had.
- But the data raise the issue as to the relationship between symptoms suggestive of Covid-19 and having a positive swab test
A bit of basic epidemiology!
- In the table below, it shows that individuals in the population who are tested can be divided into one of four groups based on their (i) presence or absence or symptoms and (ii) whether they are positive or negative on the swab test
- Government advice is that only those who would be in the yellow or blue boxes would turn up for testing
- That would mean the people in the pink box would be missed
- Also, as those in the blue box would be tested “unnecessarily”, we should aim to reduce the proportion in that group
- Therefore, there are the twin challenges of:
- capturing the ‘pink’ people
- refining what counts as symptoms to reduce the proportion of ‘blue’ to ‘yellow’ people
Do we know how what proportion of those with Covid-19 don’t have symptoms?
- Yes we do!
- This is because (and similar studies have been done in many other countries) a random population has been giving swab tests each week and answering questions on symptoms-thereby avoiding any biases of just looking at people who turn up at test sites
- In the UK less than half of this random population of households had symptoms of Covid-19 in the week before or after their positive test
- What is interesting is that this proportion had not really changed over the first few months of the pandemic
- The graph below right the proportion with symptoms has been constantly below 40% (The grey areas represent that there is a range in the likely true result because of the relative small numbers)
- Testing only symptomatic people will miss a substantial proportion of cases
Can we refine our criteria for those with symptoms to increase the likelihood they have Covid-19?
- We have a problem with Covid-19 which some viruses do not present
- This is child with chicken pox, a disease caused by a virus
- The rash is distinctive and we don’t need to identify the virus from a swab to confirm the diagnosis
- This is a person with a viral illness, it could be Covid-19, but it could be influenza, or other minor illness
- The challenge is that their symptoms from Covid-19 vary between patients and overlap with other viral diseases
Are there patterns of symptoms that should be used to prioritise for testing?
- The symptoms that were initially publicised of fever and persistent dry cough clearly overlapped with similar symptoms of other respiratory viral infections.
- Although, in some people these symptoms were for sure more severe than say seasonal flu, it was not possible to differentiate. Given limited testing capacity at that stage, those with mild symptoms were assumed to have the disease and told to isolate, as were their close contacts
- In what has been a ground breaking study, a team led by Tim Spector from King’s College London*, with a US tech partner, introduced an app for the general public to give daily updates of their symptoms and to note whether they had tested positive for Covid-19
- Around 4 million have now signed up in the UK in addition to a sizeable number in the USA
- What this enabled the researchers to address is how specific were the symptoms of Covid-19
- Their results are shown as a graph below
- To explain, just focusing on the red squares, the square opposite loss of taste and smell is associated with a 6 fold increase. What that means is that for people with that symptom they were 6 times more likely to report a positive Covid-19 test.
- In that sample 2/3 of those with a positive test had this sensory loss
- Compare that with the very modest increases in fever and cough – of course that doesn’t mean that Covid-19 doesn’t give coughs and fevers. More that these are very common symptoms and are just as likely to be associated with non-Covid-19 causes eg other viruses such as flu or just ‘having a cough’
- Almost certainly if there had been more details about the cough and the fever, say how high the temperature etc, then the risk scores would have been higher.
- But this perhaps explains how the simple public health messages about cough and fever lead to a large number of tests but only a small number of positive results.
- Just a word of caution about all these results, the percentages and risks will change depending on how common say Covid-19 and seasonal flu are at the time of the research
*Tim was my first PhD student, but I can’t claim any credit for his later career success!
Conclusion
- There are no easy answers and this blog is not about politics
- But there are some clear conclusions
- Testing only those with symptoms will miss a sizeable number of cases
- Although there is no justification for testing random people (apart from research purposes) there are groups who although asymptomatic should be prioritised for testing including:
- Contacts of cases
- Health and care workers
- Teachers etc
- Loss of taste and smell is by far the most specific symptom of Covid-19. Indeed, given that, and especially in the presence of any other symptom or if a close contact of another person with confirmed Covid-19 , there may be a case for not testing and just assuming the person is positive, and they should isolate.
- I am drawn to the conclusion that testing people with other symptoms, especially when they’re mild, is not helpful and should, given the constraints of testing, not be a priority compared to the groups listed above
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Making sense of CoVid-19 with Silman 