Table 2

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

VariableElementsRange (variation/extremes)
Population density
  • People per km²

  • People per household

  • Indoor space (m²) per person

Big European/USA/Chinese citiesRural 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, EuropeSub-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 areasSlums
  • Climate

  • Urbanisation rate

  • Air traffic intensity

  • Population movements

  • Road networks

Europe, USASub-Saharan Africa, Brazil, India
Pre-existing immunity
  • Prior exposure to other coronaviruses, BCG vaccine, and so on.

  • Non-specific immunity?

High-income countriesLow-income countries in tropical climates
Genetic factors
  • ACE variability

  • HLA variability

  • ACE, angiotensin-converting enzyme; HLA, human leucocyte antigen.