  OBJECTIVES: Estimates of the basic reproduction number (R0) of COVID-19 vary
across countries. This paper aims to characterise the spatial variability in R0
across the first six months of the global COVID-19 outbreak, and to explore
social factors that impact R0 estimates at national and regional level.
  METHODS: We searched PubMed, LitCOVID and the WHO COVID-19 database from
January to June 2020. Peer-reviewed English-language papers were included that
provided R0 estimates. For each study, the value of the estimate, country under
study and publication month were extracted. The median R0 value was calculated
per country, and the median and variance were calculated per region. For each
country with an R0 estimate, the Human Development Index (HDI), Sustainable
Mobility Index (SMI), median age, population density and development status
were obtained from external sources.
  RESULTS: A total of 81 studies were included in the analysis. These studies
provided at least one estimate of R0, along with sufficient methodology to
explain how the value was calculated. Values of R0 ranged between 0.48 and
14.8, and between 0.48 and 6.7 when excluding outliers.
  CONCLUSIONS: This systematic review provides a comprehensive overview of the
estimates of the basic reproduction number of COVID-19 globally and highlights
the spatial heterogeneity in R0. Higher values were recorded in more developed
countries, and countries with an older population or more sustainable mobility.
Countries with higher population density had lower R0 estimates. For most
regions, variability in R0 spiked initially before reducing and stabilising as
more estimates became available.
