People have jumped on practical solutions like washing hands and wearing masks in an attempt to limit the spread of COVID-19. But one question we’ve not seen answered is: where are these protective methods most important? Or put another way, where’s the riskiest place to catch COVID-19, and where should we be taking the most care to limit transmission?
As you read this, you’re probably sitting in the riskiest type of place for catching COVID-19.
Analysing Locations of COVID-19 Cases in China
A set of researchers from China collected and analysed cases across the whole country between 4th January and 11th February. In total, they identified 7,324 COVID-19 cases. 1,245 of these cases occurring in 318 outbreaks which involved 3 people or more. Using each patient’s history of exposure, the scientists were able to separate the cases into six categories, based on the locations of infection:
After analysing the cases, the scientists found the majority of the cases occurred in homes (80%) with shopping and entertainment venues being the locations with the fewest cases.
Note: beady eyed readers might note that the total number of cases in the chart above adds up to 416, not 318. This is because some of the 318 outbreaks were attributed to possible multiple locations.
But wait? Aren’t outdoor locations missing from the analysis?
Good spot! In fact, none of the 318 outbreaks of 3 or more people analysed actually occurred outdoors. They all occurred indoors. This lead the scientists to make a bold statement that the transmission of respiratory is an indoor phenomenon.
Many people may have expected this. We’re normally more cramped indoors, and there’s a higher likelihood of us touching surfaces which may have viruses. However, the data shows the scale to which indoor transmission accounts for all COVID-19 cases. In this research, it’s 100%.
The Reasons why 100% of Transmissions Occurred Indoors
The CDC defines ‘exposure’ to COVID as being within 6ft (~2m) of a person for at least 15 minutes. These are much more common inside. When indoors, we’re typically closer together (often much less than 6ft), spending long periods of time with each other.
COVID-19 can be spread through fomites – objects such as towels, tables, door handles etc. – which carry the virus. The sharing of common spaces also increases the likelihood of these objects being touched by multiple people.
Data also shows that the COVID-19 coronavirus can be COVID-19 can be transmitted in the air, typically in the form of droplets or aerosols. COVID-19 droplets are water droplets 5-10nm in diameter, that travel less than 6ft or 2m in distance. Aerosols are smaller yet, and can travel further distances.
It makes sense that in more confined environments, with limited air recirculation, these virus droplets and aerosols can travel in greater quantities.
When outdoors, any viral loads is likely to be diluted more quickly in the large volume of air. Lower viral loads mean a smaller likelihood of virus transmission.
Were There Really no Outdoor Transmission Cases?
The 318 outbreaks (covering 1,245 cases) analysed by the scientists involved outbreaks involving 3 or more people. The scientists also did a preliminary analysis of all cases, including those involving just 2 people. In total, this represented 7,324 cases of COVID-19.
Out of these 7,324 cases, the scientists found just one outdoor outbreak, involving two people in a rural village in Henan. This involved a 27-year-old man who had a conversation outdoors with an individual who had the onset of COVID-19 symptoms.
Does This Data Mean There’s No Need to Wear Masks Outside?
The single outdoor case involving a 27-year-old man who had an outdoor conversation with a person who had COVID-19 shows that masks may still prove useful in preventing outdoor transmissions. However, the data strongly suggests
This piece of research covers just 318 outbreaks of COVID-19 across China. It also covers a period when many Chinese were at home with their families (during the Chinese New Year festive period). More research for a wider range of countries, and over a longer time period would help validate these findings.
Paddy is the CEO of Smart Air, running operations from Beijing. He has a Masters in aeronautical engineering from Bristol University, UK having specialised in aerodynamics. An advocate for open data, free information and transparent business, he spends his spare time promoting honest business and social enterprise.