Workplaces, restaurants and public transport networks may need to invest in better ventilation systems and risk management processes to prevent the spread of Covid-19, as research has highlighted growing evidence for long-distance airborne transmission.
The team of researchers from the Health Security Agency and the University of Bristol set out to evaluate the potential for long-distance airborne transmission of SARS-CoV-2 in indoor community settings and to investigate factors that might influence transmission.
A review of 18 existing studies had already indicated that airborne transmission of Covid-19 from an infectious individual to others located more than two metres away can happen in different indoor non-healthcare settings.
However, the evidence from these studies was deemed to be of very low certainty. Yet, at the same time, since the early stages of the pandemic and so-called ‘super spreader’ events, there has been growing evidence suggesting that airborne transmission virus in places with poor ventilation can help to spread infection.
It is generally accepted that short-range transmission (in other words, less than two metres distance) can occur through both droplets and aerosols in the air. But there is a lack of agreement about the importance of long-distance (or more than two metres) airborne transmission in indoor settings, such as hospitality venues, leisure facilities, workplaces, or apartment blocks.
Risk managing Covid-19
For this latest research, the team reviewed studies published between January 2020 and January 2022, focusing on 18 observational studies of Covid outbreaks in countries across Asia, Europe, Oceania and the USA.
These outbreaks happened within various community settings, including apartment blocks, quarantine hotels, restaurants, buses, a food processing factory, a courtroom, a fitness facility, and during singing events.
The authors concluded that long-distance airborne transmission was likely to have occurred for some or all transmission events in 16 out of 18 studies. It was unclear in the other two studies.
In the 16 studies, one or more factors were likely to have increased the odds of long-distance airborne transmission. These were in particular insufficient air replacement, directional air flow, and activities associated with increased emission of aerosols, such as singing or speaking loudly.
In 13 studies, individuals who were likely to have been the source of infection were reported as being asymptomatic, pre-symptomatic, or just starting to have symptoms at the time of transmission.
The authors conceded this latest review did have some limitations, however. For example it only looked at epidemiological observational studies of real-world human-to-human transmission events and did not include wider evidence from environmental or experimental studies.
In addition, most of the outbreaks occurred before vaccine rollouts and so it is unclear how these results apply to populations with a high level of immunity to infection, either naturally acquired or vaccine mediated.
Nevertheless, the review critically assessed the likelihood of long distance airborne transmission of Covid-19 using a range of evidence from epidemiological data, genomic analysis and video surveillance to analysis of seating arrangements and environmental hypothesis testing, the BMJ has said.
As such, the authors concluded: “This rapid systematic review demonstrates the existing evidence that long distance airborne transmission of SARS-CoV-2 can occur in indoor settings such as restaurants, workplaces, and venues for choirs, and identified factors such as insufficient air replacement that can contribute to transmission.
“These results support the role of mitigation measures in indoor settings, such as adequate ventilation,” they added.
In a linked editorial to the research, a leading consultant microbiologist has said the findings have highlighted the need for an ‘indoor air revolution’, the BMJ has added.
Dr Stephanie Dancer, consultant microbiologist at Edinburgh Napier University, said: “The evidence presented in this review, tenuous as it is, validates the premise that tiny respiratory particles containing SARS-CoV-2 freely transmit throughout inadequately ventilated environments.
“It is hoped that public health leaders will develop practical guidance and ’tilt’ people and places closer to safety,” she concluded.