The global dissemination of COVID-19 through two coexisting international transmission patterns

The novel coronavirus SARS-CoV-2, commonly referred to as COVID-19, triggered a global pandemic. Although understanding the nature of international infection spread is critically important, extracting diffusion networks from observational data is challenging due to the inherent complexity of the phenomenon.In this study, we investigate the global infection process, including time delays, using worldwide infection case data collected from January 3, 2020, to December 31, 2022. We analyze the data using complex Hilbert principal component analysis, which captures not only concurrent relationships among variables but also leading and lagging dynamics. We then examine interactions among countries in relation to six factors: geography, population, GDP, stringency of countermeasures, vaccination rates, and government type. The results reveal that two primary trends coexisted during the period: one in 2020 and another in 2021 and 2022, with their dominance alternating over time. Specifically, in 2020, European, high-income, and democratic countries led the first trend and were typically associated with higher transmission levels.In contrast, in 2021 and 2022, African and American countries, particularly those with lower income levels, exhibited leading trends. We also find that, while internal countermeasures may have helped suppress domestic case numbers, they did not influence the trend of international spread. Furthermore, although vaccination became widespread in 2021, it likewise did not alter the pattern of international transmission.