COVID-19 circulated undetected for a few months before the first cases in Wuhan, China
Using molecular dating tools and epidemiological simulations, researchers at the University of California, San Diego School of Medicine, along with colleagues at the University of Arizona and Illumina, Inc., estimate that the SARS-CoV-2 virus probably circulated undetected for almost two months before the first human cases of COVID-19 were reported in Wuhan, China at the end of December 2019.
“Our study was designed to answer the question of how long SARS-CoV-2 could have circulated in China before it was discovered,” said senior author Joel O. Wertheim, PhD, an associate professor in the Division of Infectious Diseases and Global Public Health, UC San Diego School of Medicine.
“To do this, we combined three important pieces of information: a detailed understanding of how SARS-CoV-2 spread in Wuhan, the genetic diversity of the virus in China, and various reports of the first cases of COVID-19 in China. By combining these disparate samples, we managed to set an upper limit in mid-October 2019 for the moment when SARS-CoV-2 started circulating in the Hubei Province”.
Cases of COVID-19 were first reported in late December 2019 in Wuhan, located in the Hubei Province in central China. The virus spread rapidly beyond Hubei. Chinese authorities have surrounded the region and implemented mitigation measures at the national level. By April 2020, local transmission of the virus was under control, but at that point COVID-19 had already turned into a real pandemic, with more than 100 countries reporting numerous cases.
SARS-CoV-2 is a zoonotic coronavirus that is thought to have spread from an unknown host to humans. Numerous efforts have been made to identify when the virus began to spread among humans, based on the investigation of cases of COVID-19 diagnosed early. The first cases – and the oldest sequenced SARS-CoV-2 genomes – were associated with the Huanan seafood market area, but the study’s authors say it is unlikely for that to be the start of the pandemic, as the oldest documented cases had nothing to do with the market.
Based on this, the researchers estimate that until November 4, 2019, there would have been only one person infected with SARS-CoV-2 in China. Thirteen days later, there were four people and only nine on December 1, 2019. The first hospitalizations in Wuhan with a condition later identified as COVID-19 took place in mid-December.
This study simulated how long the virus could have circulated before giving rise to the pandemic, and yielded some surprising results. It was observed that more than two thirds of the simulated epidemics in the study went extinct. This means that if we could go back in time and repeat 2019 a hundred times, the SARS-CoV-2 virus would have gone extinct 70 times, without starting a pandemic.
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