Two days ago, Seattle-based public health researchers released a new study as a preprint* suggesting that COVID-19 cases spread throughout communities within Washington state for 4-6 weeks before the first community-acquired case had been identified.
This is deeply concerning, because it suggests that by the time the first case is identified in a community it has already gained too strong a foothold to stop the spread through self-isolation of the people who have been diagnosed or have symptoms.
On January 19, 2020, one individual was identified in Washington state who had traveled to Wuhan and contracted the disease.
The first community-acquired case was not identified in Washington state until February 28.
SARS-CoV-2, the coronavirus that causes COVID-19, develops a genetic mutation about once every fifteen days. This allows researchers to compare the genomes of different samples and build an evolutionary tree.
They analyzed 326 genomes, and found that 85% of them fell into a closely related group known as a “clade” that most likely shared the January 19 case as a common ancestor.
When calculated based on the evolutionary tree alone, the median estimate for the first spread from the common ancestor is February 1, with 95% confidence that it occurred some time between January 18 and February 9. This is consistent with the January 19 case being the common ancestor, but it cannot rule out other possibilities. For example, the January 19 case could have been closely related to a different unknown case that arrived in Washington in the weeks that followed and began the spread.
Their results also suggest that in the weeks before the first community-acquired case was detected, the number of cases doubled every 2.4-5.1 days, with a median estimate of doubling every 3.4 days.
Seven genomes from the Grand Princess cruise ship were sampled, and all fall into the same clade. Their results cannot rule out that someone from the cruise ship started the Washington outbreak, but the authors consider it more likely that someone from the Washington outbreak brought it to the cruise ship.
These results suggest that by the time COVID-19 is identified in a community, it has already been spreading for 3-4 weeks and doubling every few days. This is particularly concerning because, as a Vox article from ten days ago pointed out, rural communities are likely to get hit later, but harder. The virus will spread much more quickly in somewhere like New York City, but NYC has lots of hospitals, health care workers, organizational infrastructure, and political clout. Many small communities have none of these things.
This makes a strong case for being proactive about hygiene and social distancing (and, in my opinion, antiviral nutrition and immune support) even if you live in a community that doesn't appear to have been hit yet, or that was hit recently but where things don't seem to be that bad.
Stay safe,
Chris
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Disclaimer
I am not a medical doctor and this is not medical advice. I have a PhD in Nutritional Sciences and my expertise is in conducting and interpreting research related to my field. Please consult your physician before doing anything for prevention or treatment of COVID-19, and please seek the help of a physician immediately if you believe you may have COVID-19.
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Footnotes
*Preprints are studies destined for peer-reviewed journals that have yet to be peer-reviewed. Because COVID-19 is such a rapidly evolving disease and peer-review takes so long, most of the information circulating about the disease comes from preprints.