Cases analysis data from China suggests that by far the riskiest environment for COVID-19 is where most people are right now. We probed the data on where the riskiest places are to catch COVID-19 and where should we be taking the most caution.
People have jumped on practical solutions like washing hands and wearing masks in an attempt to limit the spread of COVID-19. But sometimes it doesn’t come down to masks and hand washing; it comes down to location, location, location.
Searching for the Riskiest Environment for COVID-19
As you read this, you’re probably sitting in the riskiest type of place for catching COVID-19. A set of researchers from analyzed 7,324 cases of spread between three or more people across China between January 4th and February 11th. The researchers broke those cases into six categories based on where people got infected:
Most cases (80%) occurred in homes and 34% in transport. Shopping and entertainment had the fewest cases.
But wait, home and transport alone add up to more than 114%. How can that be? That’s because some of the 318 outbreaks were attributed to multiple locations.
Why Are Outdoor Locations Missing?
If you noticed that none of these locations are outdoors, good spot! None of the 318 outbreaks 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. However, the data shows the scale to which indoor transmission accounts for all COVID-19 cases. In this research, it’s 100%.
Why 100% of Transmissions Occurred Indoors
The CDC defines ‘exposure’ to COVID as being within 6ft (~2m) of another person for at least 15 minutes. That is much more common inside. When indoors, we’re typically closer together (often much less than 6ft), spending long periods of time with each other.
One thing that is more common indoors is fomites. Fomites are virus particles on objects like towels, tables, door handles. Sharing common spaces increases the likelihood that multiple people will touch these objects.
Data also shows that COVID-19 can be transmitted in the air through droplets or aerosols. COVID-19 droplets are water droplets 5-10 nanometers large that usually travel less than 6 feet. Aerosols are smaller yet and can travel further distances. In confined spaces with limited air recirculation, these virus droplets and aerosols can travel in greater quantities before getting dispersed.
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 scientists first analyzed group outbreaks–those with 3 or more people. But next they analyzed cases of just 2 people (7,324 cases). 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.
Case data from China suggests that transmission is far more likely indoors than outdoors.
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. It’s also worth noting that this data covered January and February, when much of China is cold, and people spend little time outside.
This research also covers Chinese New Year, when many people in China were at home with their families. More research for a wider range of countries and over a longer time period would help validate these findings.
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