How you will get COVID-19 Delta, according to science

Percent of cases attributed to community encounter (e.g. meeting in a lift), living in same household and dining. My plot of data extracted Wang et al. 2021

A recent in-depth analysis of a COVID outbreak in Guangzhou (China) shed light on the ways people tend to get infected by COVID. Note that this is in the context of a city of 15 million inhabitants.

The most common transmission scenarios were dining and same-household at about 30% each.

Isn’t it surprising that dining is comparable to being in the same household? I imagine dining is a fairly short exposure of one or two hours whereas being in the same household you would have 8-16 hours a day plus sharing common items.

Why dining?

This was puzzling until I realised that over meals people will mask-off and as they talk and laugh, droplets will get distributed into the air. Indeed, another study showed that a COVID-infected person can expel into the air 1.59 million copies of COVID genes per hour simply by breathing and coughing. This cloud of viral particles will suspend in the air for a few minutes, giving ample time for dining partners to inhale and be infected. (For those who are interested, that study filtered out the droplets from COVID patients and used Polymerase Chain Reaction to amplify and quantify the number of copies COVID RNA that were present in the air).

If we extract the data from the Guangzhou study and count the number of people infected by each case, we see an interesting dynamic emerge.

My histogram of spreading rate reading off the data from Wang et al. 2021

First, there were many who did not spread COVID to another person. This is partly due to the lockdown that Guangzhou imposed, but even the early cases had a lot of “zero spreaders”. I imagine this is because some people’s immune systems were able to fight off the virus quickly, but also maybe those people may not have had such an active social/business life and were not dining frequently with big groups.

In contrast, there were some “super-spreader” individuals who injected between 10 to 20 others! These were mostly through dining, so I imagine that they were either very infectious or very social with big or frequent meals. The “middle-spreaders” between 1 and 5 were partly meals and partly same-household.

How comparable are these findings for your city?

All these findings are for a city in China. Although Guangzhou’s population is 15 million, its population density is only 2,500 people/km² (Wikipedia). In comparison, Singapore averages 8,358 people/km² and NYC averages 10,431 people/km² (Wikipedia).

However, dining sizes of 5–10 are common, especially if you are in a line of work that requires entertainment. So I think these findings are applicable to most countries.

Summary and Lessons

Dining is risky if COVID is rampant. This data seems to suggest those having smaller social circles and with less frequent large-group dining will be safer. If you’re an extrovert, it may be time to embrace your introversion, or perhaps concentrate your time to fewer individuals.

The effect of reducing dining sizes or frequency is clear. It is possible to break the chain of infection and reduce the reproduction rate of the virus. A full lockdown may not be necessary, but definitely some scaling back on group dining. So, do consider how to reduce the size and frequency of your dining with friends, at least while the Delta variant is rampaging outside your door.



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HealthTech Scientist who wants to change the world by helping everyone discover their best selves. Once accused of making people think too much.