DELVE group publishes evidence paper on the use of face masks in tackling Coronavirus (COVID-19) pandemic

04 May 2020

Face masks could offer an important tool for contributing to the management of community transmission of Covid-19 within the general population, according to Data Evaluation and Learning for Viral Epidemics (DELVE). The multi-disciplinary group convened by the Royal Society supports a data-driven approach to learning from the different approaches countries are taking to managing the pandemic.

The group’s analysis suggests that the use of face masks, including home-made cloth masks, could reduce onward transmission if widely used in situations where physical distancing is not possible or predictable such as busy public transport, shopping and other potentially crowded public or workspaces. This contrasts to the standard use of masks for the protection of wearers. This is thought to be particularly important because a significant fraction of infected people are asymptomatic or presymptomatic and will not know that they are infectious.

The report highlights that in parallel with any policy recommending the use of homemade face masks for the general public, it will be necessary to take all steps to ensure sufficient supply of surgical masks as well as other PPE for frontline NHS workers.

The paper was submitted to the Government’s Scientific Advisory Group on Emergencies (SAGE).

DELVE welcomes the Prime Minister’s comments last week on the usefulness of face coverings and looks forward to hearing more details on Government plans.

Venki Ramakrishnan, Chair of the DELVE committee said; “While social distancing and  maintaining hygiene, such as washing hands, remain of prime importance to reducing transmission, the evidence shows that, if used widely and correctly, face masks, including home-made cloth masks, can help to reduce viral transmission in situations where social distancing is difficult to maintain consistently. Such situations include busy public transport, shopping and other potentially crowded public or workspaces.  The British public has shown a great willingness to take action to reduce the spread of this terrible pandemic and I am sure they would take this latest additional step. The more people who do this, the more effective it will be in helping us protect each other.

“The Government has to take many factors into account when making policy and it remains essential that NHS and other frontline workers who are risking their lives on our behalf must have access to medical face masks. Our top priority should be to protect the NHS and care workers, so the public should absolutely not be rushing out to get surgical masks. However, the scientific evidence shows that even home-made masks can help to reduce the spread of the virus. 

“Many people may have the virus before symptoms kick in and some people will carry the virus without even knowing it.  Every time they cough, sneeze, talk or even breathe, they are spreading droplets containing the virus.  Covering your face can reduce the risk of infecting others but we emphasise that this is an added tool to bring down the infection rate, and not a replacement for handwashing and social distancing.”

Key points in the paper include:

  • Asymptomatic (including presymptomatic) infected individuals are infectious. Without mitigation, the current estimate is that 40%-80% of infections occur from individuals without symptoms. Universal screening of asymptomatic SARS-COV2 in women admitted for delivery in New York City shows that 13.7% were infected, and that asymptomatic women accounted for 88% of infected individuals in the study.  Of individuals who do become symptomatic, viral loads are the highest in the presymptomatic and early symptomatic phase, decreasing thereafter.
  • Droplets from infected individuals are a major mode of transmission. This understanding is the basis of the recommendations for physical distancing, and of the PPE guidance for healthcare workers. Droplets do not only come from coughing or sneezing: in a/presymptomatic individuals, droplets are generated via talking and breathing.
  • Face masks reduce droplet dispersal. Cloth-based face masks reduce emission of particles by variable amounts, for example Anfinrud et al showed that they are almost completely eliminated. Davies et al showed that cloth masks filtered viral particles during coughing at about 50 to 100% of the filtration efficiency of surgical masks, depending on fabric, with absolute filtration efficiencies of 50-70%, and about 70-80% for oral bacteria. van der Sande et al showed 50% filtering efficiency for airborne particles. One non-randomised study showed that mask use to prevent onward transmission significantly reduced viral respiratory tract infection in immunocompromised patients.

DELVE’s report states that the evidence supports the conclusion that widespread face mask adoption in situations where social distancing is difficult to maintain consistently can help to control the Covid-19 epidemic by reducing the shedding of droplets into the environment from asymptomatic individuals. This is consistent with the experiences of countries that have adopted it. 

The report also addresses implications of the science for policy makers:

  • Strategies to transition out of lockdown need to take into account the role of both symptomatic and asymptomatic individuals in spreading Covid-19. Face masks can reduce viral transmissions from such individuals. Both commercially available and homemade cloth masks and surgical masks can play a role.
  • Face masks may play an important role in situations where social distancing is not possible or unpredictable. These situations include public transport, stores and shopping areas, workplaces, within households, clinics, hospitals, care-homes, social care, and busy pavements. If used widely and correctly and on a risk basis, face masks, including homemade cloth masks, can reduce viral transmission: benefits of use increase where exposure risk is high and are marginal where it is low.
  • Public health interventions that involve cost to the public and access to reliable information tend to be taken up faster, more widely or more effectively by higher socio-economic status groups. If the use of face masks in public is made mandatory or highly recommended, interventions may be necessary to ensure that all members of the public have access to these masks and information about proper use, including guidance on washing and re-use appropriate to the domestic environment to ensure that masks themselves do not become sources of infection.
  • While there is a suggestion that use of masks might make individuals engage in more risky behaviour, at a population level the introduction of safety measures like HIV prevention measures, seatbelts and helmets have led to increased safety and even increased safety oriented behaviour. There is no evidence that individuals will engage in risky behaviour amongst the public during epidemics.
  • Clear instructions, that they should be worn in addition to other Government measures like physical distancing and handwashing, and that they primarily protect others rather than the wearer, will be necessary to support correct use and avoid risky behaviour. The establishment of standards for homemade face masks, as has been done for other areas of public health, is one approach to achieving this.
  • In parallel with any policy recommending the use of homemade face masks for the general public, it will be necessary to take all steps to ensure sufficient supply of surgical masks as well as other PPE for frontline NHS workers.

The full report can be accessed at the DELVE initiative github:


DELVE is an independent group convened by the Royal Society. Its views do not necessarily reflect the views of the Royal Society

The Royal Society is providing a secretariat for DELVE