Background: It is well established that SARS-CoV-2 is readily transmitted in indoor environments; however questions remain about the relative importance of different transmission mechanisms, the risks associated with non-clinical indoor environments and activities and the role of ventilation and plumbing systems in mitigating or amplifying transmission. This rapid evidence review identifies and integrates evidence from three distinct disciplines: epidemiology, microbiology and fluid dynamics.
Objectives: This review addresses the following research questions:
1. What evidence is there for aerosolised transmission?
2. What evidence is there for faecal-oral transmission?
3. What evidence is there regarding the role of ventilation systems in indoor transmission?
4. What evidence is there regarding the role of plumbing systems in indoor transmission?
5. What evidence is there regarding transmission via different indoor surfaces (materials and specific objects)?
6. What evidence is there for the transmission in indoor residential settings?
7. What evidence is there for transmission in indoor workplace settings?
8. What evidence is there for transmission in other indoor settings (social, community, leisure, religious, public transport)?
9. Do particular activities convey greater risk (e.g. shouting, singing, eating together, sharing bedrooms)?
10. What evidence is there for the appropriate length of distancing between people?
Data sources: Searches were conducted in PubMed, medRxiv, arXiv, Scopus, WHO COVID-19 database, Compendex & Inspec.
Eligibility criteria: We included studies reporting data on any indoor setting except schools, any indoor activities and any potential means of transmission. Additional screening criteria were modified slightly for the three study disciplines. Title and abstract and full text screening was conducted by a single reviewer, with rejections assessed by a second reviewer.
Data extraction, quality appraisal and data synthesis: We used Joanna Briggs Institute and Critical Appraisal Skills Programme tools for evaluating epidemiological studies and developed bespoke tools for the evaluation of study types not covered by these instruments. Data extraction was limited to a minimum set of items, given the rapid nature of this review. Data extraction and quality assessment were conducted by a single reviewer. We conducted a meta-analysis of secondary attack rates in household transmission. Otherwise, data were synthesised narratively.
Results and conclusion: Literature searches yielded a total of 1573 unique articles. After screening and quality assessment, fifty-eight articles were retained for analysis. Experimental evidence from fluid mechanics and microbiological studies demonstrates that aerosolised transmission is theoretically possible; however, we found no conclusive epidemiological evidence of this occurring. The evidence suggests that ventilation systems have the potential to decrease virus transmission near the source through dilution but to increase transmission further away from the source through dispersal. There is compelling evidence that, if infectious virus is present in faeces, defective plumbing systems can amplify transmission within high-rise buildings. However, although viral RNA can be detected in stool samples, we found no evidence for the detection of live infectious virus in faeces, nor did we find any examples of faecal-oral transmission. Laboratory studies suggest that the virus survives for longer on smooth surfaces and at lower temperatures. Environmental sampling studies have recovered small amounts of viral RNA from a wide range of frequently touched objects and surfaces; however, we did not find any such studies which had successfully cultured live virus from collected samples and epidemiological studies are inconclusive on the extent of fomite transmission. We found many examples of transmission in settings characterised by close and prolonged indoor contact, such as households (pooled secondary attack rate 11%, 95 % confidence interval 9, 13), communal residential settings, workplaces and religious gatherings. There were insufficient data to evaluate the transmission risks associated with specific activities; however workplace challenges related to poverty, such as continuing to work whilst ill, especially when attendance is incentivised, warrant further attention. Finally, the physical properties of droplets generated by coughing, speaking and breathing are well characterised and reinforce the importance of maintaining 2 metres social distance to reduce droplet transmission.