Background: The coronavirus disease 2019 (COVID-19) became a global pandemic within several months after it was first reported at the end of December,2019.Countries in the Northern Hemisphere have been affected the most,including the United States and European countries.Contrary to the common knowledge that infectious diseases are more prevalent in low-and middle-income countries,COVID-19 appears to affect wealthy countries more.This paper attempts to quantify the relationship between COVID-19 infections and levels of economic development with data from the U.S.and Europe.Methods: Public domain data on the confirmed COVID-19 cases during January 1 and May 31,2020 by states and territories in the U.S.and by countries in Europe were included.Incidence rate was estimated using the 2019 total population.COVID-19 cases were associated with 2019 gross domestic product (GDP) using regression models after a logarithmic transformation of the data.The U.S.data and European data were analyzed separately,considering significant heterogeneity between the two.Results: A total of 2 451 691 COVID-19 cases during a 5-month period were analyzed,including 1 787 414 from 50 U.S.states and territories and 664 277 from 28 European countries.The overall incidence rate was 5.393/1000 for the U.S.and 1.411/1 000 for European countries with large variations.Lg (total cases) was significantly associated with lg (GDP) for U.S.states (=1.2579,P ＜ 0.001) and European countries (=0.7156,P ＜ 0.001),respectively.Conclusion: This study demonstrated a positive correlation between COVID-19 case incidence and GDP in the United States and 28 European countries.Study findings suggest a potential role of high-level development in facilitating infectious disease spread,such as more advanced transportation system,large metropolitan cities with high population density,better domestic and international travel for businesses,leisure,and more group activities.These factors must be considered in controlling the COVID-19 epidemic.This study focuses on the impact of economic development,many other factors might also have contributed to the rapid spread of COVID-19 in these countries and states,such as differences in national and statewide anti-epidemic strategies,people's behavior,and healthcare systems.Besides,low-and middle-income countries may have an artificially low COVID-19 case count just due to lack of diagnostic capabilities.Findings of this study also encourage future research with individual-level data to detect risk factors at the personal level to understand the risk of COVID-19.