Moray has hit the headlines as the region has experienced “uncontrolled, sustained community transmission” (Chris Littlejohn, deputy director Public Health, NHS Grampian, quoted by the BBC). Is this due to a sudden rapid rise in infections? Or is it a result of testing more asymptomatic people?
Firstly, what do the numbers show? Here are the daily cases for Moray compared with the surrounding regions of Highland, Aberdeen City and Aberdeenshire for comparison. These data are from the NHS OpenData website run by Public Health Scotland.
Rates of positive cases have decreased in the three neighbouring regions but increased in Moray. This is encouraging, as it shows the effect is confined to Moray. The smoothed fit line for the Moray data (the bold line in blue) shows that there are less than 10 new cases a day once the peaks and troughs of the testing data are smoothed out. This rate appears to increase after 5 April.
Where does the rise in cases come from? Is it Pillar 1 (hospital) or Pillar 2 (community) testing?
The majority of tests are done in Pillar 1 (hospitals and care homes) in all four regions. The main source of the rise in Moray is in Pillar 2 (community) testing. Note that there was also a rise in Pillar 2 testing rates in Aberdeen City and Aberdeenshire in March, but this has since reduced. Pillar 1 testing has remained steady across the four regions (excepting the sudden increase in Highland region in February).
Is the amount of Pillar 2 testing in Moray unusually high? The next graph shows Pillar 2 testing per 100k population each day, and helps to show if there is an unusually large amount of testing being done in Moray’s population. For context, the populations of the regions are:
Aberdeen City – 228670
Aberdeenshire – 261210
Highland – 235830
Moray – 95820
These are from the National Records of Scotland (Mid-Year Population Estimates, Mid-2019).
Moray has had an increased rate of Pillar 2 testing from March onwards. Aberdeen City and Aberdeenshire had increased rates from March onwards, peaking at the start of April and reducing after that to below 100 tests per day per 100k people. The rate of testing in Moray has increased to above 100 tests per day per 100k population, and looks set to increase beyond that.
What about the percentage of positive tests in each Pillar? How do these compare?
Around 10% of Pillar 2 tests are positive in Moray. In the other regions, less than 5% of Pillar 2 tests have returned positive results. In all regions, Pillar 1 is returning almost zero percent positive results.
Given this detailed look at the data, what is going on? The main points are:
- There isn’t much more Pillar 2 testing per 100k population going on in Moray than in surrounding areas – but the rate of testing is increasing.
- The percentage of positive Pillar 2 tests in Moray is higher than elsewhere.
- Pillar 1 testing accounts for the majority of tests carried out each day in all regions, but returns the fewest positive results.
Another factor to consider is: who is being tested in Moray? BBC News reported:
[Jillian Evans, head of health intelligence at NHS Grampian] said the recent surge in infections in Moray was worrying because they were not linked to a “known chain of transmission”, with many of the cases also asymptomatic.
More asymptomatic testing is being done than before. This probably accounts for the majority of new cases – people are being encouraged to come forward for testing if they have no symptoms at all. People with other symptoms including diarrhoea, sore throats and headaches are also being encouraged to come for testing. Asymptomatic testing is done using lateral flow devices – however, a review of LFDs and other rapid SARS-CoV-2 tests said “it is unclear whether the tests are able to detect lower antibody levels likely seen with milder and asymptomatic COVID-19 disease.” Confirmatory testing on all lateral flow positives is done using Pillar 2 PCR – we do not yet know the sensitivity and specificity of PCR in Pillar 2 testing, so we do not have accurate estimates of how many (if any) of these new cases are true positives.
It looks like the rise in positive SARS-CoV-2 test results in Moray is down to the strategy of testing people who have no symptoms of COVID-19. If more people are encouraged to come forward for asymptomatic testing, will Moray ever come out of Level 3 restrictions? A more evidence-based approach would be to only test those who have symptoms. That way, the sensitivity of lateral flow device testing would increase, and we would have a more accurate estimate of who truly has COVID-19 in Moray.