Why do some COVID-19 patients infect many others, whereas most don’t spread the virus at all?

Updated: Jun 5

Interesting article in Science discusses how SARS-CoV-2 seems to spread in clusters through super-spread events, and the implications of this for viral control.

Here are the key points:

  • SARS-CoV-2 has many documenting cases of superspread events from ships, nursing homes, prisons, restaurants, churches, hospitals, ski resorts, meat packing plants, zumba classes.

  • This could be seen as good news, since it means that maybe our targeted mitigation efforts can be focused on preventing superspread events, and lifting other unnecessary restrictions, e.g. outside activities.

  • They discuss a measure called the dispersion factor, k, for which low values indicate much of the disease spread comes from a small proportion of cases.

  • They mention papers that suggest the dispersion factor is between 0.10 and 0.20, meaning 10-20% of cases produce 80% of new infections.

  • Evidence suggests respiratory and aerosol transmission is the primary mode, and some individuals expel more particles, especially when singing, talking loudly, or breathing deeply and heavily when doing intense exercise.

  • A Japanese study suggests indoor enclosed areas are 19x more likely to spread the virus than outdoor settings.

This article brings up a lot of interesting thoughts. I have been saying for a while that one of the key targeted mitigation strategies we should be focusing on is preventing superspread events. These happen indoors, in crowded conditions, with people unmasked, and sometimes with loud talking, singing, deep breathing or something else that causes many respiratory particles to be expelled. From what I've seen, cool, humid environments (e.g. meatpacking plants) are conducive to spread (take note friends in Texas -- as the heat brings you in to cool, humid indoor environments in AC). Some type of contact tracing would also help. These can be prevented without doing the total lockdowns that have been the strategy of choice in most places.

As is quoted in the article: “Shutdowns are an incredibly blunt tool,” he says. “You’re basically saying: We don’t know enough about where transmission is happening to be able to target it, so we’re just going to target all of it.”

However, the flip side of this is that if superspread events from small numbers of people is a primary mode of infection, how can we prevent these? This raises the stakes of mitigation, since there can be a community where the infection is under control that quickly becomes a hot spot if a superspread event happens -- and how can this be prevented? I guess with this virus we just have to accept this can happen -- but take the steps to minimize its probability and decrease the frequency of occurrence.

It also highlights the importance of having a responsive testing and tracing system in place -- to try to nip the effect of superspread in the bud before it fans out more broadly.

Tricky stuff. Hopefully this accruing information can lead to the construction of sustainable and reasonably effective mitigation strategies.

UPDATE 6/5/2020: There is another paper that was put online as a preprint that looks in detail at cases in Hong Kong, a location in which detailed tracking and tracing was done thoroughly, so they could identify/estimate the likely source of infection in many people. This paper found similar results, suggesting by their estimates that 80% of cases are caused by 20% of infected "super spreaders". Again, this should inform the construction of targeted mitigation strategies, of which a primary concern should be to prevent settings conducive to super-spread events. In doing this, it may be possible to lift other unnecessary restrictions, allowing more return to normalcy of life for many people, and still limit the viral spread.

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