Reproduction number (R) in propagation mode for two weeks

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Tired of corona numbers? – Part 2

In the first part of “Tired of Corona Numbers?” among other things, it was about the reliability of various RKI numbers. Because this topic has gained unexpected explosiveness due to the fact that the number of reproductions (R) has remained above “1” for two weeks, here is a small lookup.

The number of reproductions, which is determined by the RKI with a lot of effort, is a key factor in making trends in the infection process visible. If R is below “1”, Covid-19 decreases; above “1” the virus spreads. Currently, an almost four-month relaxation phase, briefly interrupted by the Tönnies cluster, seems to have come to an end, and that R values ​​fluctuate around 1.18: Every infected person passes on the Cov-2 virus to an average of 1.18 people.

A reasonably constant number of reproductions causes exponential growth, and accordingly the number of new infections has increased on average by around 3.5% per day for the past three weeks. Around 50,000 new infections are expected in August (there were about 15,000 new infections in July). In the past, such very rough projections often turned out to be coffee grounds reading, but we can assume that similar scenarios will be played out at the RKI and that the buzzword “exponential growth” will ring the alarm bell with regard to autumn. If there is no turnaround and the political sector is soon to wake up from the summer break mode, an expanded catalog of measures and the corresponding counter-reactions can be expected in the near future.

Nobody can rule out that we are experiencing the beginning of a longer phase of rising. However, speculations starting from the very big wave with a further increase in the R value do not currently have any data.

The current development shows a constant R-value somewhat above “1”. This may have dramatic consequences for individual high-risk patients, but for the time being is not a serious threat to the German health system and medical care.

Or is everything completely different and the worrying numbers are harmless paper tigers because the current R increase is due in particular to the increasing number of tests? The test events have actually caused considerable distortions in recent months. However, this has hardly any impact on the current situation, because the proportion of positive tests is currently relatively constant.

Another potential source of error, which caused discussions in the forum of the first article, is still in the room, and even Health Minister Spahn has it addressed: “Because we […] have brought the numbers down like this, we currently have a positive test of less than one percent […]. And we have to be careful now that we don’t go through extensive testing afterwards […] have too many false positives. Because the tests are not 100 percent accurate, but also have a small but also an error rate. “

In my article it said: “How can a published test result from currently 0.6% positive with expected 1.4% – 2.2% false positives filter out the real positives? There may be a laboratory answer to this question, but a clarifying statement from the RKI would be helpful. “

That the error rate of different PCR tests (the published RKI numbers are largely based on PCR tests, not on antibody tests) critical discussion is a matter of course for a laboratory test. But to optimize Covid-19 diagnoses there is actually a simple laboratory option that at least greatly lowers the false-positive rate: You do a double test in which PCR tests are carried out at at least two sites in the SARS-CoV-2 genome (Dual Target Test).

It is crucial, however, whether such a double test is mandatory or is used consistently. Exactly this does not seem to be the case. The RKI has not yet answered my inquiries, but there are concrete ones complaints from practice, which calls for a uniform double test: “It cannot be that the results of the simple test will be accepted in other districts (…). We therefore need a nationwide procedure.” Another source reported: “The demand from some laboratories also showed that they only test on one gene region.”

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