Table 2

Contextual variables potentially influencing transmission of severe acute respiratory syndrome coronavirus 2

Variable	Elements	Range (variation/extremes)
Population density	People per km² People per household Indoor space (m²) per person	Big European/USA/Chinese cities	Rural areas Megacities with urban slums
Social demography	Age structure: proportion of elderly and children Household composition: nuclear versus multigenerational Mixing patterns, including mode of transport Forms of religious and social events	Italy, Europe	Sub-Saharan Africa, Southeast Asia
Social practices	Mode of greeting and social contact (shaking hands, kissing, hugging, etc.) Handwashing, water and sanitation Ventilation and air conditioning	High-income countries or areas	Slums
Geography	Climate Urbanisation rate Air traffic intensity Population movements Road networks	Europe, USA	Sub-Saharan Africa, Brazil, India
Pre-existing immunity	Prior exposure to other coronaviruses, BCG vaccine, and so on. Non-specific immunity?	High-income countries	Low-income countries in tropical climates
Genetic factors	ACE variability HLA variability