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Early Spread of COVID19

Many tested negative for the flu in January and February. Could this have been COVID19?

Short answer: Yes.
Long answer…

We have to outline the first “original” cases:

• Jan 21-Feb 23: U.S. detected 14 COVID19 cases all related to travel from China

• Feb 26: 1st non-travel US case confirmed in CA (patient was ill starting Feb 13)

• Feb 28: 2nd non-travel US case popped up in WA

So, from this timeline, COVID19 started spreading in the United States on Feb 26 right? Nope.

CDC found that it was spreading earlier in the US by looking at four things:

1. Seattle Flu Study: Some scientists, during this time, just happened to be conducting a Seattle Flu Study. They were basically monitoring the flu from Nov 2018- March 2020 by testing people randomly. After the pandemic started, they went back to test these samples for COVID19. From Jan 1-Feb 20, none of their tests were positive. Their first sample was positive on Feb 21; the following week there were 8 positives; the following week there were 29 positives.

2. Gene analyses: Genes from early cases suggest that a virus imported directly or indirectly from China began circulating in the US between January 18 and February 9, followed by a COVID19 strain from Europe.

3. CDC has found other cases before Feb 26

• Jan 31: CA women became ill. Died 6 days later. She did not travel internationally. Postmortem, COVID19 positive.

• Feb 11: An infected passenger boarded the Grand Princess in Seattle leading to two outbreaks.

• Feb 13: CA man died at home. He did not travel internationally. Postmortem, COVID19 positive

4. Surveillance: ER records did NOT show an increase in visits for COVID19–like illness until February 28. CDC thinks this is because there were too few people with the disease to see an increase in ER visits in a meaningful way.


Translation? Community spread of the Chinese COVID19 strain likely started in January. Community spread of the European COVID19 strain started in Feb.


Why do we care? There are many reasons. One being that we can better estimate how many people truly died from COVID19 that were missed. We know from excess death analyses that we missed a lot in the beginning, meaning our current numbers are underreporting.


Love, your local epidemiologist

Data Source: Jorden MA, Rudman SL, et al. Evidence for Limited Early Spread of COVID-19 Within the United States, January–February 2020. MMWR Morb Mortal Wkly Rep 2020;69:680–684. DOI: http://dx.doi.org/10.15585/mmwr.mm6922e1external icon

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