The recent lifting of some the lockdown measures in the UK now allows people to visit non-essential retail outlets which have been closed since the late March, provided that they maintain adequate social distancing, which is currently set at 2m.
However, there have been calls from various factions of society to reduce this distance with critics arguing that current distancing will be virtually impossible to observe, especially in the hospitality sector. In addition, there does not appear to be a universally accepted social distance. Advice from the World Health Organization suggests a distance of at least 1m,1 and although several countries including China, Denmark and France have adopted this measure, others, for example, Australia, Germany and Italy suggest 1.5m whereas the UK, Canada and Spain have decided upon 2m. In the UK, the government has consistently maintained that policy decisions are informed by the science. But what is the nature of the science behind social distancing measures and how good is the evidence upon which they are based?
Efforts directed towards reducing the spread of an infectious disease primarily seek to contain the community spread of the infection through non-pharmacological approaches including hand washing, travel restrictions, school closures which are collectively referred to as mitigation strategies. Such an approach is designed to buy time before the development of an effective treatment such as a vaccine and cannot continue indefinitely. One aspect of any mitigation strategy, is social distancing which has proven to reduce the rate of infection as witnessed in a systematic review of 15 studies which found that workplace social distancing reduced the cumulative attack rate of influzena by 23% especially when combined with other mitigation strategies.2 However, the authors noted that there were few epidemiological studies conducted in actual settings and called for more research to assess the effectiveness of social distancing. Nevertheless, while social distancing is undoubtedly an important strategy for minimising the spread of a virus, a more relevant and practical question is what is the minimum effective distance required, especially given the variation in the rule adopted in different countries.
The ease with which a virus can spread depends on the different routes of transmission. Clearly direct contact, for example, touching an infected individual can lead to infection but many infections are spread through droplet and airborne contact. Droplet infection occurs when the infected material is carried through the air inside the expelled mucosalivary droplets created after someone coughs, sneezes or talks and these subsequently enter the eyes, mouth or nose of another person. These droplets can also settle on surfaces, providing a further source of infection, referred to as fomite transmission. Fortunately, the relatively large size of mucosalivary droplets means that they can only travel through air for short distances such as 1-2m, a fact established in the 1930s.3 In contrast, as expelled droplets transition from the warm, moist environment of the lungs into the colder atmosphere, water evaporates from the droplets, leaving residual particulates, known as aerosols, which travel much further distances through the air and enter the body via the airways. Clearly then, infection becomes more likely when in close proximity to an infected individual. But how far can the coronavirus travel in the air? There is still uncertainty over this crucial point. Some studies have found evidence of viral RNA more than 2m from infected patients4 whereas other have found no virus 2 to 5m from infected patient’s beds.5 Early evidence from Wuhan, China suggested that the virus could be detected in air up to 4m from infected patients.6
With such uncertainty and a desire to reduce the social distance to the minimal level required for safety, a recent study published in the Lancet sought to provide some much-needed clarity on the person-person transmission of COVID-19.7 The authors analysed data from 172 observational studies and 44 comparative studies in both health-care and non-healthcare settings with a total of 25,697 patients. Due to a small number of studies with COVID-19, the authors combined data on the middle east respiratory syndrome (MERS), severe acute respiratory syndrome (SARS) with any COVID-19 studies. Their analysis revealed that when comparing distances of >1m vs <1m (that is, no social distancing), the risk of infection reduces from 12.8% to 2.6%, that is, keeping at least 1m apart reduces the risk of infection from approximately 13% to 3%.
The researchers also found that “for every 1m further away in distancing, the relative effect might increase by 2.02 times”. This provides crucial support for the current 2m social distancing rule in the UK. It suggests that by reducing this distance from 2m to 1m, the risk of infection would double. This latter point is reinforced in the paper when the authors state that “our current review supports the implementation of a policy of physical distancing of at least 1m and, if feasible, 2m or more”. The study also revealed that the use of masks reduced the risk of infection by 85% though this was only more effective in healthcare settings and that eye protection reduced the risk by 78%.
While the finding from the Lancet paper offer support for the current UK recommendation, it is important to recognise that the study had some limitations. For instance, the authors were unable to assess the effect of exposure duration since this factor varied considerably in studies up to an hour. Another limitation was that not all studies give precise distances and no study quantitatively evaluated whether distances of more than 2m were more effective.
In summary, and with no obvious end in sight for the current pandemic, governments around the world face the unenviable challenge of balancing the health and economic needs of their countries. While policy decisions are the responsibly of government, the current easing of lockdown measures is likely to make social distancing much harder to maintain in practice. In such circumstances, it would seem sensible, based on the conclusions of the Lancet study, to ensure that face and eye protection becomes mandatory, not just on public transport, but in all premises to help minimise the risk of infection.
- World Health Organisation. Coronavirus disease (COVID-19) advice for the public. www.who.int/emergencies/diseases/novel-coronavirus-2019/advice-for-public.
- Ahmed F, Zviedrite N, Uzicanin A. Effectiveness of workplace social distancing measures in reducing influenza transmission. BMC Public Health 2018;18(1):518.
- Wells WF. On air-borne infection: study II. Droplets and droplet nuclei. Am J Epidemiol 1934;20(3):611-18.
- Santarpia JL et al. Transmission potential of SARS-CoV-2 in viral shedding observed at the University of Nebraska Medical Center. medRxiv 2020; published online March 26. DOI:10.1101/2020.03.23.20039446.
- Faridi S et al. A field indoor air measurement of SARS-CoV-2 in the patient rooms of the largest hospital in Iran. Sci Total Environ 2020;725:138401.
- Guo ZD et al. Aerosol and surface distribution of severe acute respiratory syndrome coronavirus 2 in hospital wards, Wuhan, China, 2020. Emer Infect Dis 2020; Apr 10;26(7): doi: 10.3201/eid2607.200885
- Chu DK 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. Lancet 2020; June 1. https://doi.org/10.1016/ S0140-6736(20)31142-9.