A new study on SARS-CoV-2, the novel coronavirus that causes COVID-19, suggests that the virus was circulating undetected at least two months before the first cases were detected. The first cases of COVID-19 in China were reported during late December of 2019. The study, published in Science was conducted by a group of researchers from University of California, San Diego, University of Arizona and Illumina inc.
The researchers also claim to have found in their study that the mutating virus is self-limiting, which means that it dies out naturally more than three quarters of the time without being able to cause an epidemic.
Joel O.Wertheim, the senior author of the study from the Department of Medicine, University of California, San Diego, commented about the study and its motives, “Our study was designed to answer the question of how long could SARS-CoV-2 have circulated in China before it was discovered.”
"To answer this question, we combined three important pieces of information: a detailed understanding of how SARS-CoV-2 spread in Wuhan before the lockdown, the genetic diversity of the virus in China and reports of the earliest cases of COVID-19 in China. By combining these disparate lines of evidence, we were able to put an upper limit of mid-October 2019 for when SARS-CoV-2 started circulating in Hubei province,” he further said.
The first cases of COVID-19 were reported in Wuhan, China. The virus then rapidly spread beyond Wuhan and the Hubei province. Chinese authorities were very quick in responding to the health threat and within four months, by April 2020, the spread of the virus was brought under control. However, by then the virus had spread to over 100 countries and causing a pandemic.
Earlier analyses have revealed that the SARS-CoV-2 virus may have been linked to the Wuhan meat market. This was based upon the early genome sequencing of the virus and the initial clusters of cases. However, the authors of the new study say that it is unlikely that the virus’s spread started from the meat market because the early cases had no direct link to the market.
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In the new study, the researchers used evolutionary analysis of the virus to decipher when the first case of SARS-CoV-2 occurred. The researchers estimated that the median number of people infected with SARS-CoV-2 in China was not even one till November 4, 2019. Thirteen days from this date, the cases were estimated to be four and by December 1, it became nine. Reportedly, the first hospitalisations in Wuhan happened in mid-December of 2019.
The authors of the study used a variety of analytical tools in order to model how the SARS-CoV-2 virus could behave at the early phase of the pandemic, at a time when the virus was an unknown entity. The tools included epidemic simulations which are based on the biology of the virus, for example its transmissibility and other factors. The inference that came out of such analysis was that in merely 29.7 % of the cases the virus was able to create a self-sustaining epidemic. In about 70% cases, the virus could infect relatively lower numbers of people before dying out.
Commenting on the approach of the study and the inferences, Joel Wartheim was quoted to have said, “Typically, scientists use the viral genetic diversity to get the timing of when a virus started to spread. Our study added a crucial layer on top of this approach by modelling how long the virus could have circulated before giving rise to the observed genetic diversity.”
He further added, “Our approach yielded some surprising results. We saw that over two-thirds of the epidemics we attempted to simulate went extinct. That means that if we could go back in time and repeat 2019 one hundred times, two out of three times, COVID-19 would have fizzled out on its own without igniting a pandemic. This finding supports the notion that humans are constantly being bombarded with zoonotic pathogen.”
The authors wrote in the paper that the original strain of the novel coronavirus caused a pandemic because it was dispersed in wide range of regions which allowed the virus to persist. Especially, the virus could thrive in urban areas where transmission was easier. The study revealed that in rural areas, where population density is less, the epidemics became extinct in 94.5 to 99% cases.
Actually, pandemic surveillance system was proven to be not sufficient, the study reveals. On this aspect Wertheim said, “Pandemic surveillance wasn't prepared for a virus like SARS-CoV-2. We were looking for the next SARS or MERS, something that killed people at a high rate, but in hindsight, we see how a highly transmissible virus with a modest mortality rate can also lay the world low."