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Infectious Diseases

Updated 19 August 2020

Retroactive Covid-19 testing indicates numbers in Wuhan and Seattle initially far higher

Retroactive testing reveals that there were many Covid-19 cases in Wuhan and Seattle - where the first US case was confirmed - at the beginning of 2020.

  • A study estimates that Covid-19 started spreading in cities months before the first confirmed cases
  • Researchers analysed flu data from Wuhan and Seattle from samples that were retroactively tested for the virus
  • Their estimates far surpass confirmed cases at the beginning of 2020 – without even accounting for asymptomatic cases

The Covid-19 pandemic officially started in Wuhan, China, on 31 December 2019.

Less than a month later, the first US case was confirmed in Seattle.

However, a study published in EClinicalMedicine posits that the numbers were initially in actual fact much greater than thought – and that the outbreak started much earlier than the first confirmed case.

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Following the flu trail

The researchers analysed two studies from the respective cities, which retroactively tested swabs for people initially thought to have the flu in hospitals.

In Wuhan, four out of 26 patients presenting flu-like symptoms tested positive for the coronavirus prior to 12 January 2020.

In Seattle, 25 cases tested positive for the coronavirus out of 2 353 children and adults who also presented with flu-like symptoms before 9 March. Using these datasets, the researchers extrapolated how many actual infections there were in each city.

They also compared these findings to positive flu-only cases, as the two viruses "[overlap] natural histories and modes of transmission". 

"Thus, we expect that once SARS-CoV-2 got a foothold in a city, spreading across multiple communities, its geographic and demographic patterns might mirror those of influenza," argue the researchers.

More accurate estimates

For Wuhan, they estimate that there were 1 386 Covid-19 cases in people over the age of 30 between 30 December 2019 and 12 January 2020, while in Seattle they estimate that 2 268 children and 4 367 adults were infected between 24 February and 9 March. 

"In Wuhan, there were roughly two symptomatic cases of Covid-19 for every three cases of influenza; in Seattle, there was one paediatric case of symptomatic Covid-19 per every nine influenza cases, and one per every seven in adults."

In comparison, only 422 cases were officially reported in Wuhan by the time China went into lockdown on 23 January. In Seattle there were only 245 confirmed cases with 36 deaths by 9 March.

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Earlier outbreak

This means that the initial outbreaks from a single source happened far earlier, with Wuhan's infection taking root between 26 October and 13 December, while Seattle's infection period likely occurred between Christmas last year and 15 January this year. 

And these estimates are only for symptomatic cases – asymptomatic cases will remain unknown and could have reached levels of over 10 000 cases in each city. The same is noted for children – who tend to experience much milder Covid-19 symptoms if infected – which could have also numbered in the thousands.

"If we assume 50% of infections are asymptomatic, then we project there may have been over 15 000 undetected Covid-19 cases at the time. We further estimate that the Seattle epidemic originated with cases that arrived infected around 6 January, 2020."

Covid-19 was 'unseen'

The researchers reiterate that their study doesn't mean the numbers were ignored by healthcare professionals, rather that they just didn't know what they were looking at in the early stages of the pandemic. 

The researchers also highlighted some limitations. Their data was sparse and the two cities had differing testing methods that could have varied in accuracy. Their findings also hinge on the assumption that flu and Covid-19 spread in a similar fashion, and the seasonality of flu can have an impact on numbers, as Covid-19 hasn't shown seasonal attributes. 

"With these caveats in mind, we conclude that our method provides a way to roughly triangulate the unseen emergence of the Covid-19 pandemic in cities around the world during the early months of 2020."

It also highlights how difficult it is to truly know the real numbers of Covid-19 and that we shouldn't become too complacent when it looks like numbers are decreasing. 

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Image credit: Pixabay