LAUSR

Ruan et al. (2022) published in National Science Review (NSR) an analysis that concludes two early centers in the spread of COVID-19 [1]. Since the strain (referred to as DG1111) that spread globally to cause the pandemic has never been found in Wuhan prior to its arrival from outside Asia in March 2020, Wuhan is a local center, rather than the ‘global epicenter’. Citing observations from Europe [2–8], we suggest that the jump from animal hosts to humansmust have been earlier than the fall of 2019 topermit the evolution from the ancestral DG0000 strain to the global DG1111 (0 and 1 designating the ancestral and derived variant, respectively). This essay is written in response to two recent back-to-back publications in Science [9,10] that contradict our conclusion. In Worobey et al., Wuhan with its seafood market is concluded to be the epicenter of the COVID-19 pandemic. In Pekar et al., the appearance of SARS-CoV-2 in humans is estimated to be in November of 2019, presumably in Wuhan. We will first discuss the flaws of the two Science publications and expand the scope of our original conclusion as other recent papers have largely corroborated our conclusion. The flaw in Worobey et al. is pointed out by Cao et al. Since Worobey et al. used only samples from Wuhan to pinpoint the epicenter of COVID-19, they could not possibly have concluded otherwise. The contradiction with our conclusion is hence easily explained. Similarly, Pekar et al.’s interpretation is also conceptually flawed. They estimated tMRCAs of SARS-CoV-2 found in humans, which means the time to the most recent common ancestor of the viral variants. However, the authors interpret tMRCA to mean the timing of the viral jump from animal hosts to humans. Imagine that an ancestral SARS-CoV-2 invadedhumanpopulations, say, 50 years ago. Since then, the viral population has undergone a series of genetic changes due to genetic drift and selective sweep. The continual losses and gains of genetic diversity may result in the re-building of the extant diversity only three months ago, which is the tMRCA. Therefore, the most recent common ancestor is usually much younger than the time of invasion. Using the logic of Pekar et al., we would have concluded that humans separated from chimpanzees about 0.5 M years ago, which is the tMRCA of human genomic variations, instead of the 6 M years commonly accepted. The two issues we have to discuss are (i) the invasion of the virus such as SARS-CoV-2 from animal hosts to humans; and (ii) the subsequent spread of the virus in human populations. For viral pathogens, the place of origin (referred to as PL0 for Place zero) is usually conflated with the beginning of the spread (PL1, Place of the first epidemic) [11]. The reasons and evidence from prior epidemics for separating PL0 and PL1 have been extensively discussed [11–16]. Generally, PL0 would more likely be the countryside or wildlife preserve where human density is low but, for years, local humans have been in frequent contact with animal hosts. In contrast, PL1 should be a settlement of high human density that facilitates human-to-human transmission after the virus arrived from PL0. PL1 thus corresponds to the general understanding of the ‘epicenter’ of pandemics. We now consider the arrival of the virus from PL0 to PL1 when it is ready to invade. Distance between sites may not be a determining factor, given that humans and bats (the most likely mammalian hosts for their high-density habitation) are highly mobile. There are, nevertheless, empirical observations that epidemics are highly stochastic events. Using the branching process of probability, Ruan et al. (also see Kucharski et al. [17]) have shown that invasions into a new population could only sporadically trigger local epidemics in the early phases of an epidemic. Local epidemics may even reach a moderate level of infections before fadingout on its own. In this sense, there may be many false PL1’s (that rise and fall) preceding a true PL1 that eventually establishes itself and succeeds in exporting the epidemic far and wide. The analyses in Ruan et al. [1] focus on the very first new strain of SARSCoV-2 which emerged in late 2019. This new strain bears 4 mutations of the DG group (C241T, C3037T, C14408T and A23403G; the latter one being the D614G amino acid change). The new strain (or haplotype), designated DG1111, replaces the ancestral DG0000 as it has a much higher fitness than DG0000 or each of the intermediate haplotypes. The replacement was rapid and DG1111 quickly became the foundation of the global pandemic by March–April of 2020. Importantly, the haplotype DG1111 did not exist in Asia until March of 2020 and remained far less common in Asia than in Europe for several more weeks [18,19], which indicates that DG1111 originated outside of Asia, likely from Europe. Nevertheless, it may have been believed that DG1111