The more data from patient cases available to research, the better it learns to understand the pathogen SARS-CoV-2 and Covid-19 disease. The focus here is on the question of how much and from when corona infected people are contagious, i.e. pose a danger to other people.
Virus multiplication study at the moment of infection
Charité chief virologist Prof. Dr. Christian Drosten knows about the latest research findings; some of them were won under his direction. In an issue of “Corona Update” podcasts on NDR he reports from a recent study from Hong Kong. 96 cases were analyzed and the same observations were made as by his own team in a small investigation a few weeks ago: that the virus multiplied immediately after infection.
The coronavirus SARS-CoV-2 is “so detectable in the earliest smears,” Drosten reports that it is smeared “in the first two days after smears”. This means that the peak of the virus must be before the first day. ”With each subsequent smear, the virus occurrence would decrease.
But – what does that mean for infectivity?
In the same context, the research group examined the point at which corona infected people are contagious. Together with scientists from Guangdong, they looked at cases of pairs of people, some of which infected the other.
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The focus was on how much time passed between the onset of symptoms in one person and the onset of symptoms in the other affected person. This period, according to Drosten in the technical term “serial interval”, could be determined relatively precisely: with an average of 5.2 to 5.8 days.
Corona people are contagious almost immediately
Prof. Drosten compared these new numbers with those from a previous, “very well done study” from the same university. The focus was on the incubation period, i.e. the time between infection and the first appearance of symptoms or onset of the disease. An average incubation period of 5.2 days was defined.
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“This is of course interesting,” says the specialist in the NDR podcast, “because we have a phenomenon here where the series interval, the series length, is practically as long as the incubation period.”
Consequently, on average, a patient waits for symptoms for as long after infection as the person he infected. He could have passed on his own infection immediately afterwards.
This is the case for almost half of those infected
As Drosten explains, a frequency peak for infectivity can be calculated based on the probability distribution of the available data. “And the so-called Area under the curve, the area covered by this probability curve before the onset of symptoms, is 44 percent. That means you can assume that 44 percent of all infection events took place before the person infected was even ill. ”
Against this background, the current distance rules (= one should keep a distance of at least 1.5 meters from people who do not belong to the same household) make all the more sense. Most would automatically stay away from obviously sick people. Apparently, however, they were / were already infectious far before – or even far more infectious.
All current knowledge subject to change
The virology professor makes it clear that this is a respectable group of researchers. Nevertheless, scientific knowledge is currently particularly quickly released to the public or the specialist press (at least online on a so-called preprint server). This step would normally have to be preceded by appraisal processes that can take weeks or months, but at the moment there is a lack of time and probably also capacity.
At the moment it is “sometimes from a scientist to a journal”, without detours. It is therefore not uncommon that after a while – if updated knowledge has been gained – appropriate adjustments are necessary. It could also be that the point at which corona infected people are contagious can be specified again.