key: cord-0962754-e0pbs354 authors: Leung, Nancy H. L. title: Transmissibility and transmission of respiratory viruses date: 2021-03-22 journal: Nat Rev Microbiol DOI: 10.1038/s41579-021-00535-6 sha: 2fec47cdad7720ac3eef8f1060828f44e119e06f doc_id: 962754 cord_uid: e0pbs354 Human respiratory virus infections lead to a spectrum of respiratory symptoms and disease severity, contributing to substantial morbidity, mortality and economic losses worldwide, as seen in the COVID-19 pandemic. Belonging to diverse families, respiratory viruses differ in how easy they spread (transmissibility) and the mechanism (modes) of transmission. Transmissibility as estimated by the basic reproduction number (R(0)) or secondary attack rate is heterogeneous for the same virus. Respiratory viruses can be transmitted via four major modes of transmission: direct (physical) contact, indirect contact (fomite), (large) droplets and (fine) aerosols. We know little about the relative contribution of each mode to the transmission of a particular virus in different settings, and how its variation affects transmissibility and transmission dynamics. Discussion on the particle size threshold between droplets and aerosols and the importance of aerosol transmission for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza virus is ongoing. Mechanistic evidence supports the efficacies of non-pharmaceutical interventions with regard to virus reduction; however, more data are needed on their effectiveness in reducing transmission. Understanding the relative contribution of different modes to transmission is crucial to inform the effectiveness of non-pharmaceutical interventions in the population. Intervening against multiple modes of transmission should be more effective than acting on a single mode. Tables Supplementary Table 1 ✓ 81 - ✓ 82, 83 - - - - ✓ 84 - - - - - - - - - - SARS-CoV-2: ✓ 138 SARS-CoV: ✓ 77, 139 MERS-CoV: -HCoV-OC43: -HCoV-229E: -HCoV-NL63: -HCoV-HKU1: - 160, 197, 199 h Particles with aerodynamic diameter larger than 5 µm are traditionally defined as droplets whereas those smaller are defined as aerosols. However, there is ongoing discussion on redefining the particle size threshold between droplets and aerosols (refer to the section 'Terminology and defining features of modes of transmission' in the main text). Therefore, for evidence on droplet transmission, evidence is provided in parentheses if evidence of virus recovery is only identified in particles with aerodynamic diameter between 5-100 µm. Air samples collected without size fractionation but were collected over 2 meters from a known source (for example, an infected individual) are considered as evidence suggestive of aerosols. i Evidence for virus genetic materials recovered in droplets or aerosols in human exhaled breath for IV types/ subtypes, PIV, RSV and HMPV is based on the author's own additional data of the published study 146 . j Each precautions represents a set of infection prevention and control practices and personal protective equipment recommended by the WHO for healthcare workers during routine patient care (excluding aerosolgenerating procedures) within healthcare facilities, with consideration of the current understanding on the modes of transmission of the respective pathogen. These transmission-based precautions are to be applied in addition to Standard precautions, which is always applied in all healthcare settings for all patients. 225 . - - - - ✓ 141 - - ✓ 136 - - 6 Supplementary - - - - - - - - - - - - - - - - - - - - Supplementary- ✓ 201 ✓ 151 - ✓ 158, - A: ✓ 203 A/H1N1: ✓ 167 A/H3N2: - B: ✓ 167 - - - - - ✓ 183 - ✓ 183, 184, 204 ✓ 213, 214 - - - ✓ 215-218 - - - -Alcohol denatures proteins in the presence of water 225 . -Alcohol had higher viricidal activity on enveloped than non-enveloped viruses 225 e.g. inactivating coronaviruses in 30 seconds [226] [227] [228] , although reduce viability by >1,000fold on artificially inoculated hands when for adenovirus and rhinovirus 229 . -Alcohol-based hand sanitizers more efficacious than plain soaps on pathogen inactivation in vivo, although mostly based on evidence of bactericidal and less on viricidal activity 230 . -Multiple systematic reviews suggested hand hygiene alone significantly associated with reduced respiratory illness but not influenza virus infection in community settings 79, [231] [232] [233] , perhaps due to insufficient compliance 234, 235 . -Studies on the effectiveness of hand hygiene on respiratory virus transmission in healthcare settings were not identified. -Insufficient studies to compare the efficacies of plain soaps versus alcohol-based hand sanitizers against respiratory infections 231, 233 . -Droplet, Aerosol, (Contact) -Face coverings as source control: When worn by an infected individual, reduce virus release to the environment by filtration 146 and immediate virus exposure of nearby healthy individuals by deflection 236 . -Face coverings as protection: When worn by a healthy individual, reduce exposure to virus-laden droplets and aerosols in the air. (-Might also reduce virus transfer for contact transmission by reducing the frequency of hands touching respiratory mucosa 237 .) -As source control, surgical masks efficaciously reduced influenza virus and coronavirus release from patients by filtration 146, 148 ; efficacies on exhaled droplets and aerosols may vary between viruses 146 . Studies using mannequins suggested deflection also important in reducing virus release 236 240, 241 efficacious by filtration against artificial bacteriophage 238 or influenza aerosols 239-241 challenge. -As protection against long-range transmission, in the absence of environmental air flow only 1% of radiolabeled saline aerosols generated from the source mannequin reached the exposed mannequin 3 feet apart, where only fitted respirators but not surgical masks reduced exposure to aerosols 236 . -Multiple systematic reviews of observational studies 242, 243 or randomized trials 243-245 mostly suggested the use of face coverings alone, or in combination with other NPIs, effective in reducing risk of respiratory illness or virus transmission in healthcare 242, 243 and high-risk community settings 245 , while others do not 244 . -Low compliance to face shield during high-risk procedures associated with higher risk of respiratory illnesses in healthcare workers 246 . -Face mask use by household members before the primary case developed symptoms significantly associated with reduced SARS-CoV-2 household transmission in a retrospective cohort study 138 . -Directional airflow provides clean air from the cleanest to less clean patient care areas, e.g. introducing air at the ceiling and exhausting near the floor, or via laminar airflow. -Lower ventilation associated with rhinovirus RNA detection in the air in office environment in an observational study 254 . -Multiple systematic reviews suggested strong and sufficient evidence supporting the association between indoor ventilation and airflow patterns with transmission of SARS, influenza, measles and chickenpox 255, 256 , although not yet validated by intervention studies or randomized trials 257 . -Directional airflow may reduce the risk of airborne infection in vulnerable patients or nosocomial transmission in healthcare settings 258, 259 , and also in community settings e.g. aircraft cabins 260 . -Some hypothesized increasing indoor mechanical ventilation may be less effective or cost-effective to achieve sufficient risk reduction in crowded indoor areas given high level of exposure 261 , or might increase aerosol dispersion and infection risk for individuals further away from the source 260 ; directional airflow may therefore play a more important role than ventilation in reducing transmission 262 . -Aerosol, Contact -UVGI is the use of UV light in the germicidal range (200-320 nm), especially UV-C (200-280 nm), for air and surface disinfection against bacteria 263 or viruses by nucleic acid cross-linking 264 . -Air disinfection: in upperroom UVGI, irradiation confined to room area above room occupants' heads to minimize direct exposure, but requires good vertical air movement in the room. 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