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A study by the WHO has caused confusion: Is the mortality of people infected with corona much lower than assumed? A closer look at the figures reveals a misunderstanding.
“New WHO study astonishes even experts – that’s how deadly the virus really is”: Headlines such as these could give the impression in recent days that the World Health Organization (WHO) now considers Covid-19 to be less dangerous. But this is a misconception.
What is the study about?
The current discussion is prompted by the initial research findings of the renowned epidemiologist John Ioannidis of Stanford University, which have been published on the website of the “Bulletin of the World Health Organization” – a scientific magazine published by the WHO. The publication states that the first online version of the study has been reviewed, but does not yet contain all final corrections.
For the analysis, Ioannidis had analyzed 61 studies from around the world as well as national data on Covid-19 mortality. According to this, the average infectious mortality rate of all evaluated studies was about 0.23 percent. This means that on average 23 out of 10,000 people infected with the coronavirus died.
“The derived infectious mortality rates tended to be lower than the estimates made earlier in the pandemic,” concludes Ioannidis. Initial data from China had estimated infectious mortality at 3.4 percent. In the meantime, mathematical models have assumed about one percent.
The only apparent contradiction
Some media then referred to the case mortality rate quoted by the Robert Koch Institute (RKI), which, according to Monday’s daily management report, is around 2.7 percent – almost ten times as high as Ioannidis’ figure. This can give the impression of a scholarly dispute, which does not exist at all.
Because the contradiction to the official RKI statistics is only apparent. Two parameters can be used to measure the lethal potential of a disease:
Infectious mortality, or IFR for short: It indicates the proportion of deaths measured in relation to all infections. This value is also examined in the current study.
Case mortality, CFR for short: This indicates the proportion of deaths from all – and this one word makes the decisive difference – known infections. The RKI refers to this value in its daily management report.
Since an infection with Sars-CoV-2 can proceed without symptoms and is by no means extensively tested in all countries, many infections worldwide probably remain undetected. Therefore, the case mortality rate is by definition higher than the infectious mortality rate. So why is it used at all?
It is difficult to say at present how high the number of undetected cases is. According to the WHO, twenty times more people worldwide could be infected than have been detected. However, even this figure is only a rough estimate.
Antibodies against the virus in the blood of humans could provide information about the number of undetected cases. If they are found, it is very likely that the person has been infected – possibly without noticing it himself. In Germany, an antibody study by the RKI and the German Institute for Economic Research (DIW Berlin) started in October, with 34,000 people participating. It is intended to clarify how many people in Germany have actually become infected with the virus.
Such antibody studies are not perfect either, because apparently by no means all those who were demonstrably infected with the corona virus also produce measurable antibodies. But they are currently the best chance to shed light on the dark figure.
“One death more or less that is recorded is really important”.
However, previously published antibody studies are often based on low case numbers. This can be seen in the so-called Heinsberg study, which has also been included in the current meta-analysis, which puts the infectious mortality rate at around 0.37 percent. However, the extrapolation is based on only a handful of deaths associated with corona infection.
“One death more or less that is recorded is really significant,” said Gérard Krause from the Helmholtz Centre for Infection Research (HZI) in Braunschweig after the publication of the Heinsberg study a good six months ago. He therefore recommended that the cause of death be investigated more closely. Possibly, he said, the infectious mortality rate was underestimated because hardly any nursing homes were affected.
Less than a month ago, epidemiologist Rod Jackson emphasized in a guest article for the “NZ Herald” that antibody studies are only meaningful if the sample is representative of the population. Evaluations from New Zealand and Iceland are not very helpful, as only few Covid-19 deaths have been reported there so far. “Studies with fewer than several hundred Covid-19 deaths are hardly worth looking into,” Jackson said.
Ioannidis considered studies with more than 500 samples in his current meta-analysis, not all of which – including the Heinsberg study – were representative of the entire population. So did he work in a scientifically impure way?
No fixed measured value
The aim of Ioannidis was not to determine a globally valid infectious mortality rate. This is not even possible. Infectious mortality is not a fixed value that, once calculated for one disease, remains valid worldwide. It depends on many factors.
In fact, according to initial data from China, mortality was initially estimated to be higher at the beginning of the pandemic – as Ioannidis rightly notes. At that time, little was known about Covid-19. It is now clear that a large proportion of those infected have only mild or no symptoms. In addition, they can probably pass on the virus before they even notice anything of the infection.
Ioannidis also emphasizes how much the mortality rate varies – depending on whether the elderly in particular are infected, how good the medical care is, how high the proportion of people with pre-existing conditions is in the population. For example, the infectious mortality rate in the studies he evaluated varied between zero and 1.63 percent.
Results of the representative antibody study planned in Germany are not yet available. It is therefore difficult to estimate the current level of infectious mortality in Germany.
Only recently, a research team led by Andrew Levin of Dartmouth College published results of another meta-analysis, according to which the infectious mortality rate for Covid-19 in the U.S. could be around 0.8 percent. Virologist Christian Drosten also referred to this study in the NDR podcast. Because the population in Germany is somewhat older than in the USA, Drosten considers an infectious mortality rate of about one percent for Germany or slightly higher to be plausible. “I’m not a demographer, I can only estimate that,” the virologist qualified, however.
Even though it is currently difficult to quantify the infectious mortality rate in Germany precisely, it is clear how much it can be influenced. This is also shown by the analysis from the USA. If older people are protected against infection with Covid-19, the infectious mortality rate can be more than halved – from 0.8 to 0.3 percent.
— Source: derspiegel.de, Picture: stockilyapp.com
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