Coronavirus: an estimate of the real number of infections in Switzerland

On 12 March 2020, according to the Federal Office of Public Health (FOPH), there were 815 confirmed cases in Switzerland.

There are obvious challenges to this figure. Possibly the most important is the time lag between infection and a confirmed case.

On average, there are 5.1 days between catching the virus and the onset of symptoms.

If you look at the chart above from a WHO report on COVID-19 in China, which shows new coronavirus cases from 8 December until 20 February for all of China, you’ll see there are orange and blue bars. The orange bars represent confirmed cases. The blue ones are actual cases – these are calculated by counting backwards from when the symptoms appeared, something that can only be done in retrospect.

On 24 January 2020, there were around 1,000 confirmed cases. This is the sum of all the orange bars up to and including that date. Now add up all of the blue bars up to and including 24 January 2020 and you’ll arrive at a cumulative total of more than 16,000 actual cases. In the early phase of the epidemic in China when there were 1,000 confirmed cases, there were 16,000 actual cases, 16 times the number of confirmed cases. They just couldn’t see all these cases because most patients hadn’t shown symptoms and tested positive.

A few days ago, medical professionals in Switzerland were instructed by the FOPH to reserve testing for high risk patients with symptoms, which includes those over 65 and those with certain underlying illnesses. So, many of those with symptoms are no longer tested.

In addition, some with the virus are asymptomatic or have symptoms so mild that they’re not aware they have the illness. In a number of Asian countries they tested people without symptoms that had been in contact with people testing positive. But Switzerland is no longer testing this group.

Given the patchy testing in Switzerland, the 815 confirmed cases could be much higher. If we conservatively assume that with better testing they’d have identified 1,000 cases in Switzerland, then as we saw in China, there could be well over 16,000 actual cases.

From here, the number cases in Switzerland is set to rise fast. One difference between China and Switzerland is that China appears to have contained the spread. However, there are no signs that Switzerland has managed to do the same thing. In Switzerland, there has been little of the lockdown, testing, cleaning and vigilance that contained the spread in China. 

If you look at the epidemic curve above from a recent WHO COVID-19 situation report it offers some insight into the near future. The bars in this chart are new confirmed cases outside China between 30 December and 8 March. The pink bars, which represent new cases in Europe, show total confirmed cases roughly doubling every two days from 1 March 2020. If Switzerland is on a similar curve it could easily reach 100,000 actual cases next week, if its spread is unimpeded.

If every person with the disease infects an average of two people then the disease has an Ro of 2. A WHO report on COVID-19 in China estimates an Ro of between 2 and 2.5. Other Ro estimates for the virus range from 1.4 to 3.9.

The chart below shows the rise in cases at an Ro of 2. At the beginning the rise is slow. However, then it rises steeply. From 1 to 12 in the chart below the number barely changes. From 13 onwards the rise is steep.

It is important to note that the numbers on the x axis are not days. Ro numbers are independent of time and no time span was mentioned in the WHO report.

Another curve (below) offering insight into the possible shape of the real epidemic curve in Switzerland was published by RTS based on confirmed case data from John Hopkins University and Switzerland’s FOPH.

Switzerland is marked in red. There is a notable difference between the shape of the curves in Denmark, Norway, Italy and Switzerland and those of China, Hong Kong, Singapore and Japan. South Korea is somewhat of a hybrid of these two distinct groups. It resembles the European group at the beginning and then appears to be beginning to flatten like China et al.

In the UK, they belatedly presented more realistic figures. On 12 March 2020, the UK’s chief medical officer admitted that up to 10,000 in the UK are “likely to have coronavirus”, a large number when compared to the 459 confirmed cases at the time. He also said “we’re in a period where we’ve got some [cases] but it hasn’t yet taken off.”

Jacques Fellay, a researcher at EPFL, told RTS on Saturday that he thinks there could now be 10,000 cases in Switzerland.

More on this:
WHO report on COVID-19 in China (in English)

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