Before revision:  We also analyzed the possible existence of a bias: countries with low GDP-per capita , typically located in warm regions, might have less intense testing and we discuss correlation with the above variables.
After revision:  We performed a Principal Component Analysis, in order to find their significant independent linear combinations. We also analyzed a possible bias: countries with low GDP-per capita , typically located in warm regions, might have less intense testing and we discuss correlation with the above variables.