Transmission Barriers

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This page collects observations, interpretations, and consequences action about the epidemiology of SARS-CoV2 in general. Please observe the structure of the page, when you add your content. Please use references where possible. Remember to find the relevant page. For example, if your observation is about transmission routes, please use that page, instead of posting your content here.

Face masks

What is already known=

  • Small, short-term study: Surgical masks worn by patients reduce aerosols shedding of the virus. The abundance of viral copies in fine particle aerosols and evidence for their infectiousness suggests an important role in seasonal influenza transmission.
  • Larger study: A 50% compliance in donning the device resulted in a significant (at least 50% prevalence and 20% cumulative incidence) reduction in risk for fitted and unfitted N95 respirators, high-filtration surgical masks, and both low-filtration and high-filtration pediatric masks.
  • Meta-analysis of studies: Meta-analysis of randomized controlled trials (RCTs) indicated a protective effect of masks and respirators against clinical respiratory illness (CRI) (risk ratio [RR] = 0.59; 95% confidence interval [CI]:0.46-0.77) and influenza-like illness (ILI) (RR = 0.34; 95% CI:0.14-0.82)
  • Standard surgical masks are as effective as respirator masks (e.g. N95, FFP2, FFP3) for preventing infection of healthcare workers in outbreaks of viral respiratory illnesses such as influenza. No head to head trial of these masks in COVID-19 has yet been published, and neither type of mask prevents all infection. Both types of mask need to be used in combination with other PPE measures. Respirator masks are recommended for protection during aerosol-generating procedures (AGPs). Rapid reviews on wider PPE measures, and what counts as an AGP, are ongoing. [1]
  • In a large meta-analysis performed for WHO concludes that face mask use could result in a large reduction in risk of infection [2]


Analysis and interpretation

Consequences for action

Hand Hygiene

What is already known

  • Coronavirus can be efficiently inactivated by surface disinfection procedures with 62-71% ethanol, 0.5% hydrogen peroxide or 0.1% sodium hypochlorite within 1 minute. [3]. This is good news for the Alcohol Handrug Formula that WHO prescribes. [4]
  • Coronaviruses have a viral envelope, which makes them potentially susceptible to surface-active agents such as soap and alcohol. In vitro, several hand-rub products with an alcohol content of at least 75% reduced SARS-CoV-1 virus load by at least 4 log units and wine vinegar and 70% ethanol reduced SARS-CoV-1 load by more than 3 log units. It seems reasonable to extrapolate these results to SARS-CoV-2.[5]
  • In a study of over 20,000 person-years, outpatient visits for respiratory illness were 45% lower among army recruits who washed their hand at least five times a day compared with controls. Jefferson et al reviewed physical interventions to interrupt or reduce the spread of respiratory viruses. They reported an odds ratio of 0.54 for respiratory infections in those who washed their hands frequently, compared with controls.[5]


  • Strict hand hygiene is one of the pillars of COVID19 prevention and control. [6]

Analysis and interpretation

  • The advice of strict hand-hygiene in community settings, cannot always be followed, for example in communities that have no access to safe drinking water.

Consequences for Action

  • Translation of the advice for hand-hygiene to poor resource settings is needed


  1. What is the efficacy of standard face masks compared to respirator masks in preventing COVID-type respiratory illnesses in primary care staff?The Centre for Evidence-Based Medicine. COVID-19 Evidence. March 24, 2020
  2. CHU, Derek K., et al. Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis. The Lancet, 2020.
  3. Kampf, G., Todt, D., Pfaender, S., & Steinmann, E. (2020). Persistence of coronaviruses on inanimate surfaces and its inactivation with biocidal agents. Journal of Hospital Infection.
  4. Guide to Local Production: WHO-recommended Handrub Formulations.
  5. 5.0 5.1 Hand Disinfectant and COVID-19. The Centre for Evidence-Based Medicine. COVID-19 Evidence. March 17, 2020
  6. WHO Coronavirus disease (COVID-19) advice for the public