Prediction, projection, illustration — call it what you wish, but when you make a statement about what’s going to happen next, people are going to assess whether you’re correct or not. Sir Patrick Vallance, the Chief Scientific Adviser, said last week:
“At the moment we think that the epidemic is doubling roughly every seven days...If, and that’s quite a big if, but if that continues unabated, and this grows, doubling every seven days… if that continued, you would end up with something like 50,000 cases in the middle of October per day.
Making projections is fraught with danger, but there are ways to caveat such statements that allow better understanding. For instance, it is looking gloomy today and the weather forecast says there is a 50 per cent chance of rain. Providing a probability estimate gives us a much better understanding of the likelihood of an event occurring, and it allows us to prepare; we think we’ll take an umbrella.
As we go into the winter, cases of acute respiratory infections will rise as they always do. In the last two weeks, consultations for acute respiratory infections in primary care have doubled to 86 per 100,000. In line with this, cases of Covid-19 are rising. The question is: by how much?
The way to answer this is to put the 'doubling roughly every seven days' to the test, by tracking the cases. At the moment, there is a divergence in the case data between the date of reporting and the 7 days moving average with the 49,000 cases that were projected to occur by 13 October. Forecasting the future is fraught with problems — keeping up to date and monitoring changes are essential to understand what happens next.