In a press briefing today Professors Chris Whitty and Patrick Vallance showed epidemic curves for Spain and France — demonstrating how cases numbers have been growing rapidly, possibly exponentially, since August. As is often done when using case numbers by publishing date, the raw numbers are smoothed with a seven-day moving average. Drawn this way, the data shows a continued upward trend.
But drawing the epidemic curve for Spain using cases by symptom onset produces a different result. We have put these two methods together on a single graph so that they can be compared:
The epidemic curve based on the symptom onset date does not show the same continued growth — it appears to show cases stalling in late August. As the majority of lag between the publishing date and symptom onset is between one day and two weeks (but in some cases longer) the most current data for the blue line will be revised upwards.
We repeated this exercise for England’s data:
We have previously shown that (accounting for a potential two week lag in reporting) as of 3 September, case numbers based on positive PCR tests appear to be flat in Spain. It depends, however, on which method of analysis you choose.
We believe that the date of onset of symptoms is crucial to interpret the data. In most cases, the shedding of infectious virus is all but over after 7 to 10 days from infection and 5 to 7 days from onset of symptoms. In the most vulnerable, shedding goes on for a lot longer — sometimes weeks. In addition, we have reported that Spain has three definitions of Covid, one of which does not involve laboratory confirmation.
It’s unclear which definitions are being used in the government’s brief — and how comparable the Spanish figures are with the definitions used across the UK.
While the interpretation of recent case numbers in the UK is potentially complicated by well-documented problems with testing, we are able to replicate a similar pattern in cases by date of report and by date of the specimen across the UK data. We will keep the data under review over the next few days and weeks to assess the changes in English data and observe whether cases are increasing or not.
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