Updated May 28, 2020, 5:28 p.m.
In the course of the corona pandemic, so-called superspreading events keep making headlines: In a choir rehearsal of the Berlin Cathedral Choir, around 60 out of 80 members present were infected with the virus; after visiting a restaurant in East Frisia, 20 people were infected with the virus. What role do such events play and how can you handle them sensibly? Virologist Christian Drosten deals with these questions in the current NDR podcast “Coronavirus Update”.
An infected person can infect dozens of others with the SARS-CoV-2 virus under certain conditions. Several such superspreading events have become known in the past few weeks.
To understand the importance of such situations for the overall course of the epidemic, according to the virologist Christian Drosten first clarify the terms. The fact that there are individual superspreading events does not in itself say much.
The exact definition of superspreading is also different in epidemiology: “Superspreading means that there are large outbreaks and that the small outbreaks or the small transmission chains are not very efficient at the same time, so that the entire infection epidemic is carried by superspreading events.”
The term superspreading expresses above all an inequality in the infection process – namely that some of the infectious people infect an above-average number of other patients. Another term for this is over-dispersion. “Few people infect many others, and most infect only a few or none,” said Drosten.
Superspreading is not a prerequisite for large breakouts. Even an infectious disease that is completely evenly distributed and is not subject to over-dispersion can cause major outbreaks.
Dispersion factor k describes inequality in the infection process
The degree of over-dispersion is expressed by the dispersion factor k, which is between 1 and 0. With a dispersion factor of 1, all infected people infect the same number of other people. With a dispersion factor of 0 there are big differences.
With SARS-CoV-1, the factor is approximately 0.1. This means that 73 percent of all SARS infected people had less than one follow-up case, but that 6 percent had more than eight follow-up cases. According to Drosten, such a large inequality brings advantages for the controllability of the infectious disease. Because you can concentrate on containment methods that prevent superspreading.
According to Drosten, anyone who is highly infectious not only depends on individual biological factors, but also on conditions such as social behavior and the external circumstances when the information is passed on virus. That is why Drosten sees a key to curbing epidemics in preventing social situations that encourage superspreading.
How high is dispersion with SARS-CoV-2?
With the novel corona virus, it is not yet clear how high the k factor is. Drosten cites several recent studies that come to very different results. The scientists always had the problem that the data situation was not yet solid enough to draw reliable conclusions from it.
However, a study by leading epidemiologist Gabriel Leung from Hong Kong highlights Drosten. This estimates the dispersion factor in the current pandemic at 0.45. “Unfortunately, this is a very high figure,” said Drosten. The inequality in the contagions would therefore not be particularly large.
The Hong Kong study is remarkable for another reason. Leung made the observation that a cluster – that is, a group of infected ones – does not grow if one delays the isolation of a known infection case.
Insulate if suspected
The somewhat confusing observation explains Drosten that by the time an actual infection is diagnosed, it is already too late to stop this chain of infection. Therefore, people would have to be isolated in suspected cases before it can be proven that they are infectious.
“We find someone who is infected: Has he had a social situation in the past 2-3 days that is suspicious for a superspreading event? And if so, then all people who have also been in this suspicious situation have to be infected consider and isolate immediately, “said Drosten.
Japan had followed this principle very early in the course of the current pandemic and the numbers would confirm that the procedure was successful. This strategy would also make sense for Germany in the coming weeks and months.
More and more children are returning to schools and daycare centers, where the cluster principle clearly applies. According to Drosten, this is an opportunity to try to identify clusters and isolate suspicious cases.
“If a teacher is infected, look at the classes in which he has taught in the past few days: These students must all stay at home.” According to Drosten, one-week isolation is sufficient because the infectious time with COVID-19 is significantly shorter than initially thought.
Apparently, some FFP2 masks that are not fully functional are in circulation. The Authority for Health and Consumer Protection in Hamburg warns of this. Which models are affected.
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