What's the use of studying the COVID-19 positivity rate?

When comparing the current coronavirus numbers, with numbers which where reported back in March and April, it might seem like some European countries are currently in a worse COVID-19 outbreak than back in March and April. This might happen if you only compare the number of new cases reported in March and April with the number of new cases currently being reported. However, we have to keep in mind that:

• In most of Europe there was very limited COVID-19 testing capacity compared to today and as many people with symptoms were not tested in March, the true number of coronavirus infected persons was many times higher than reported back then. Here's a simple explanation of this, which I gave back in early March: article.
• Now in September, many countries are doing 5 to 10 times more COVID-19 tests. There also seems to be better contact tracing in place, which means asymptomatic cases are nowadays being tracked more frequently and as a result more 'positive tests' are being found.

Because of the above, I prefer to look at the COVID-19 positivity rates instead of the number of new cases, to judge the situation in a country.

Current positivity rates in Europe

The 'current positivity rate' is the ratio of confirmed cases (last 7 days) to the number of performed PCR-tests (last 7 days). The exact positivity rates as calculated are given in the following bar chart.

Compared to the same bar chart of exactly 1 month ago, we can see that most countries in Europe are seeing a increase in the positivity rates:

• 22 of 33 European countries saw its current positivity rate move upwards between August 17 and September 17.
• 11 of 33 European countries saw its current positivity rate move downwards between August 17 and September 17.

The following bar chart visualizes the changes in positivity rate between August 17 and September 17.

Worth noting in the above charts:

• In general, Europe is not only seeing an increase in new coronavirus cases, but it's also seeing a increase in the positivity rate throughout Europe. This is different from the situation when there were more cases being reported, while the positivity rates remained very low (mostly below 2%).
• Ukraine already had a high positivity rate (~10%) back in August, and is now the country in Europe with the highest positivity rate: 11.7%

The positivity rate in Ukraine

• In the past four weeks, it were especially Hungary and Czech Republic, who reported a rapid increase of the positivity rate, meaning the number of new cases has been growing the fastest, relative to the amount of tests being done.
• Also Spain, France and Austria are reporting a clear increase in the positivity rate, indicating COVID-19 outbreak is growing and it is not because more coronavirus tests are being done.
• On the other side of the spectrum, Armenia used to be the country with the highest positivity rate but the positivity rate is now trending down there and was 5.5% on average during the last 7 days.
• Also some other countries who have been reporting an accelerated growth in new cases from August 17 to September 17 (e.g. Belgium, Germany) in fact did not see a rise in the positivity rate. For these countries, the recent growth in cases is fueled by the fact that more tests have been performed in recent weeks.

Additional sources used

Although most European countries report every day on testing activity, some countries only report once a week on testing activity. At this moment in time, Newsnodes only gathers testing data by countries who report on COVID-19 testing every day, so to create the above map additional I gathered additional data. Because the day that weekly testing data is released can be different for each country, some data might be a few days up to 1 week old.

Below are some examples for traceability purposes of the above data.

Source: COVID-19 Situationsberichte, RKI

Source: Point épidémiologique, Santé Publique France

Source: Veckorapport, Folkhälsomyndigheten