Table 4

Factors related to transmission patterns and severity of respiratory viruses

  • Infection efficiency, transmissibility.

  • Capacity to survive outside the human body (including in aerosols, in droplets, on surfaces, in stools, in intermediate animal hosts, etc.)

  • Potential to shed virus from an infected person, asymptomatic or diseased.

  • Genetic stability or variability (affecting the potential of long-lasting immunity).

  • Viral load determines the incubation period with the formula high load ->short incubation period ->high severity.

Human host
  • Human susceptibility to the virus; transfer of parental immunity to newborns.

  • Route and efficiency of human-to-human transmission.

  • Presence and capacity of asymptomatic carriers to transmit the virus.

  • Immunity created after infection, its robustness and how long-lasting it is.

  • Severity and duration of the disease: proportion symptomatic, lethality (CFR).

  • Pathogenicity and disease spectrum; disease pattern according to age and comorbidities, and related potential to spread.

Natural environment
  • Temperature, humidity and seasonal changes in climate affecting the stability and transmission potential of the virus and human susceptibility.

  • Increasing extreme weather conditions such as droughts and severe storms, as well as global climate change may also affect transmission patterns.

  • Air pollution may also play a role in the transmission and stability of the virus.

Human environment/social geography
  • Demographic variables such as population density, age structure and household composition.

  • Mixing patterns within households, including bed sleeping patterns, related to housing conditions and hygiene practices.

  • House construction with solid walls or permeable walls (thatched walls, straw mats).

  • Mixing patterns among households related to settlement patterns: social networks, urban–rural differences, working conditions, religious practices and commuting patterns.

  • Variables related to built environments, road infrastructure and socioeconomic conditions.

  • Mobility between communities, including international travel.

  • Crowding institutions: for example, elderly homes, extended families, boarding schools, child institutions, seclusion during tribal ceremonies, hospitals, nursing homes, military barracks and prisons.

  • CFR, case fatality rate.