Confirmation of the centrality of the Huanan market among early COVID-19 cases

Abstract: The centrality of Wuhan's Huanan market in maps of December 2019 COVID-19 case residential locations, established by Worobey et al. (2022a), has recently been challenged by Stoyan and Chiu (2024, SC2024). SC2024 proposed a statistical test based on the premise that the measure of central tendency (hereafter, "centre") of a sample of case locations must coincide with the exact point from which local transmission began. Here we show that this premise is erroneous. SC2024 put forward two alternative centres (centroid and mode) to the centre-point which was used by Worobey et al. for some analyses, and proposed a bootstrapping method, based on their premise, to test whether a particular location is consistent with it being the point source of transmission. We show that SC2024's concerns about the use of centre-points are inconsequential, and that use of centroids for these data is inadvisable. The mode is an appropriate, even optimal, choice as centre; however, contrary to SC2024's results, we demonstrate that with proper implementation of their methods, the mode falls at the entrance of a parking lot at the market itself, and the 95% confidence region around the mode includes the market. Thus, the market cannot be rejected as central even by SC2024's overly stringent statistical test. Our results directly contradict SC2024's and -- together with myriad additional lines of evidence overlooked by SC2024, including crucial epidemiological information -- point to the Huanan market as the early epicentre of the COVID-19 pandemic.

• The paper demonstrates that using an automatic bandwidth selection in kernel density estimation clearly supports the Huanan market's centrality in early COVID-19 cases.
• It refutes alternative statistical approaches by showing that improper measures distort centrality assessments, emphasizing the importance of precise methodology.
• The analysis integrates epidemiological data on linked and unlinked cases to strengthen the argument for the market’s pivotal role in the outbreak's origin.

Analysis of Centrality in Early COVID-19 Cases and the Huanan Market

The paper offers a detailed examination of the centrality of the Huanan Seafood Wholesale Market in the early stages of the COVID-19 pandemic. This analysis is framed as a response to criticisms posed by Stoyan and Chiu (SC2024), who challenged previous findings by introducing statistical tests predicated on a specific premise of centrality. The authors of this manuscript meticulously address the methodology and interpretations of SC2024, arguing against their premises and conclusions by presenting alternative analytical strategies and pointing to overlooked epidemiological evidence.

Methodological Clarifications

The authors of this paper engage in a substantive methodological discussion to refute SC2024's claims questioning the centrality of the Huanan market. SC2024 argued that using the centroid or mode as a measure of centrality and employing a bootstrapping method would provide a more accurate statistical representation. However, this paper contends that these considerations are based on erroneous premises and that their implementation suffers from technical inadequacies.

Notably, the authors highlight that a proper utilization of the mode as a measure, combined with an appropriate selection of kernel density estimation (KDE) parameters, places the center of early case locations in line with the market. Their choice to use an automatic, matrix-based bandwidth selection in KDE, rather than a simplistic circular Gaussian kernel with an arbitrary bandwidth, exemplifies a more technically rigorous approach. This adjustment demonstrates that the market falls within the 95% confidence region, negating SC2024's claims.

Epidemiological Context and Interpretation

Beyond statistical refutations, the paper emphasizes the epidemiological evidence correlating early COVID-19 cases and the Huanan market, which was not adequately considered by SC2024. The manuscript reiterates that the market was identified early on as a key location due to its association with a significant proportion of initial cases, a factor acknowledged globally since the pandemic's inception. The presence of wildlife susceptible to SARS-CoV-2 in the market, alongside dense spatial concentration of cases around it, provides a non-spatial corroboration of its centrality in the outbreak.

The authors focus on the fundamental distinction between linked and unlinked COVID-19 cases in December 2019, asserting that a spatial analysis without this differentiation results in the loss of critical insights. They highlight that unlinked cases, those with no direct epidemiological connection to the market yet residing near it, further reinforce the argument for its role as the outbreak's epicenter.

Implications and Conclusions

By meticulously challenging SC2024's statistical approach and illuminating significant lines of evidence, this paper consolidates the position of the Huanan market as a central point in the early spread of COVID-19. It underscores the complex interplay between spatial data and epidemiological evidence in understanding such pandemics.

The implications of these findings are twofold. Practically, establishing the market's centrality aids in understanding the pathways of zoonotic spillover, guiding public health strategies to mitigate similar outbreaks in the future. Theoretically, this analysis advocates for integrating multiple evidence sources in epidemiological studies, beyond mere spatial distributions, to build a comprehensive narrative of disease origins.

Overall, the paper argues effectively for the adoption of more robust statistical methods and a multidimensional analytical framework in outbreak analysis. It suggests future research directions exploring zoonotic pathways, which are essential for preempting and managing future pandemic threats.