The COVID virus is likely to spread two months before the first human case, UCSD says

March 19, 2021

Using molecular dating tools and epidemiological simulations, University of California, San Diego School of Medicine It is highly probable that the SARS-CoV-2 virus was undetected and circulated before the first human case of COVID-19 was reported in Wuhan, China in late December 2019. ..

writing Thursday online version of scienceResearchers, along with colleagues at the University of Arizona and Illumina, also said their simulations suggest that the mutant virus disappears spontaneously in more than three-quarters of the time without causing an epidemic. I am.

A case of COVID-19 was first reported in late December 2019 in Wuhan, Hubei Province, central China. The virus spread rapidly across Hubei province.

Chinese authorities have blocked the area and implemented mitigation measures nationwide. By April 2020, local transmission of the virus had been suppressed, but by then COVID-19 had already become a pandemic, with cases reported in more than 100 countries.

According to Joel Weltheim, an associate professor of infectious diseases and global public health at the University of California, San Diego School of Medicine, researchers may have spread the virus by mid-Autumn 2019 at the earliest. I found that.

SARS-CoV-2 is a zoonotic coronavirus that is thought to have jumped from an unknown animal host to humans. Much effort has been made to identify when the virus first began to spread to humans, based on a survey of early-diagnosed cases of COVID-19.

In this study, researchers used evolutionary analysis of molecular clocks to determine when the first proband of SARS-CoV-2 occurred. The “molecular clock” uses the mutation rate of a gene to estimate when two or more organisms diverge, in this case the common ancestor of all variants of SARS-CoV-2. It is a method term and is estimated in this study. It will be mid-November 2019 at the earliest.

Researchers estimate that the median number of people infected with SARS-CoV-2 in China was less than one by November 4, 2019. Thirteen days later, there were four, and on December 1, 2019, there were only nine. Hospitalization in Wuhan, later identified as COVID-19, occurred in mid-December.

The authors of the study used a variety of tools to model how the virus behaved between the first outbreak and the early stages of the pandemic. These included epidemic simulations based on the known biology of the virus, including infectivity and other factors.

In only 29.7% of these simulations, the virus could cause a self-sustaining epidemic. In the other 70.3%, the virus infected a relatively small number of people before they died. The average unsuccessful epidemic ended just eight days after the proband.

“Our approach has had some amazing results,” Wertheim said. “We found that more than two-thirds of the epidemics we tried to simulate were extinct.

“That is, if we could go back in time to repeat 2019 100 times, 2 out of 3 times, COVID-19 would have exploded naturally without igniting a pandemic. This discovery is a constant human discovery. It supports the idea of ​​being attacked, a pathogen of zoonotic diseases, “he continued.

Wertheim said that even if SARS-CoV-2 was prevalent in China in the fall of 2019, it is likely to have been prevalent at low levels, at least until December of that year.

The authors write that the original strain of SARS-CoV-2 was prevalent because it was widely dispersed and thrived in easily infected urban areas. In simulated epidemics involving less dense rural communities, epidemics had a 94.5% to 99.6% chance of becoming extinct.

Since then, the virus has mutated multiple times, making many variants more susceptible to transmission.

“Pandemic surveillance was not prepared for viruses like SARS-CoV-2,” Wertheim said. “We were looking for the next SARS or MERS, which kills people at a high rate, but later on, a moderately mortal and highly infectious virus could lower the world. You can see that there is sex. “

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