COVID-19 appeared as a new and unknown disease in Wuhan, China in late 2019.
Initial research identified that it was being spread by contact with infected people who came into close contact (>2m) with other people, or touched surfaces which others then touched, and by so doing so picked up the virus.
Public Health England (PHE) published guidance based on surface transmission: Transmission of SARS-CoV-2 and Mitigating Measures’, now available on the UK Government website.
As more evidence has emerged there has been a significant change in the basic assumptions as described above, and an understanding that the SARS-CoV-2 virus can also infect people through airborne routes which may lead to transmission at distances beyond 2m, especially in poorly-ventilated spaces.
The SAGE Environment and Modelling Group published advice to government on the potential for transmission via aerosols entitled ‘Role of Aerosol Transmission in COVID-19’.
PHE have updated their guidance as more information on COVID-19 has come available, and are now providing guidance on airborne and surface transmission. Their website is updated regularly and remains a source of the current UK Government advice including access to various sector guidance.
COVID-19 Mechanism of transmission
The SARS-CoV-2 exists inside a host person, and for a limited time, in the area immediately around that host. As the host breathes, the virus is expelled into the air in droplets and aerosols. The infected person may also have virus on their hands and they can contaminate surfaces they touch. There is also evidence of the virus RNA being expelled in faeces, however there is currently very little evidence that this contains infectious virus.
There are three main routes by which the virus is thought to be transmitted:
Close range aerosols and droplets
A person who is in close proximity (<2m) to an infected person can be directly exposed to the aerosols and droplets in exhaled breath. These can cause infection though direct deposition of larger droplets onto the eyes or mucous membranes, or through inhalation of aerosols. At close range the concentration of virus is highest and hence the risk of transmission through this route is likely to be the greatest.
The SARS-CoV-2 virus in exhaled breath is carried on the airstreams inside a building which can infect others who are more than 2m away if they breath it in. Evidence suggests that this risk is greatest where people spend a significant period of time (30 min+) in a poorly ventilated space. This route of transmission appears to be minimal in well ventilated buildings and outdoors.
Surfaces can become contaminated by droplets and aerosols falling onto surfaces or by an infected person with contaminated hands touching surfaces. If a susceptible person touches a surface that harbours sufficient virus, and then touches their nose, eyes or mouth they could become infected. The virus has been shown to persist on some surfaces for several hours under laboratory conditions, however it is not clear how long infectious virus remains on surfaces in the real-world.
Research is ongoing worldwide using outbreak data to identify routes of transmission, risk factors and to understand the relevant strategies to deal with surface, droplet and airborne transmission. Research has also studied the risks associated with each through laboratory studies such as National Institute of Allergy and Infectious Diseases work on Aerosol and Surface Stability of SARS-CoV-2 as compared with SARS-CoV-1, published in the New England journal of medicine, April 2020. This paper gives time scales for survival of virus on different materials as well as in the air.
Evidence suggests that the risk of transmission increases with the duration of time spent with an infected person and with proximity to the person. The disease is recognised to be highly overdispersed, with around 80% of infections thought to be caused by 10-20% of people. As such, there have been a number of “super-spreading” outbreaks where one person has infected multiple others in a short period of time. These are often characterised as happening in poorly ventilated and crowded indoor settings.
Singing and intensive aerobic activity are additional risk factors; both are thought to significantly increase the number of aerosols and droplets generated by an infected person. Analysis of an outbreak among the Skagit Valley Choir members in Washington, USA which infected 53 of 61 people, suggested that high aerosol generation combined with poor ventilation led to the significant outbreak
A further challenge is asymptomatic transmission, where people can transmit the virus without having symptoms, or before symptoms show. This is most likely to happen at the early stages of infection when people with the virus are at their most infectious. This means that it is not necessary for someone to have a temperature or to be coughing to spread the infection, and is why adhering to infection control and isolation guidance is so important.
A recent paper published by the Department of Applied Mathematics and Theoretical Physics and the Department of Engineering, University of Cambridge, provides the most up to date view of the Effects of ventilation on the indoor spread of COVID-19. This paper is published in the Journal of Fluid Mechanics in November 2020.
This paper reports that although the relative importance of airborne transmission of the SARS-CoV-2 virus is unclear, increasing evidence suggests that understanding airflows is important for estimation of the risk of contracting COVID-19. The data available so far indicate that indoor transmission of the virus far outstrips outdoor transmission, possibly due to longer exposure times and the decreased turbulence levels (and therefore dispersion) found indoors, and also the better survival of the virus in indoor conditions. This paper discusses the role of building ventilation on the possible pathways of airborne particles and examine the fluid mechanics of the processes involved.
At the current time there is no known medical cure against COVID-19 – by vaccine or other physiological means, and therefore the advice being given by PHE is to lessen risk, avoiding inhaling/exhaling in any air which might happen to contain the virus by wearing a mask and following government guidelines on social distancing and hand washing. In addition, a range of engineered infection controls can be applied to reduce risks and these are described in this manual.
Coronavirus is carried into a building by an infected person, who will transmit it into the airstreams inside a building by breathing, coughing and/or sneezing. The air inside the building is, therefore, carrying droplets and aerosols which contain COVID-19, which will infect others, if they breath it in. Some of these droplets and aerosols will fall onto surfaces, which then become a transmission risk. And a COVID-19 host will touch surfaces and increase transmission spread through that route.
A recent paper published in the Journal of Fluid Mechanics in November 2020, by the Department of Applied Mathematics and Theoretical Physics and the Department of Engineering, University of Cambridge, provides the most up to date view of the effects of ventilation on the indoor spread of COVID-19.
The paper reports that, although the relative importance of airborne transmission of the SARS-CoV-2 virus is controversial, increasing evidence suggests that understanding airflows is important for estimation of the risk of contracting COVID-19. The data available so far indicate that indoor transmission of the virus far outstrips outdoor transmission, possibly due to longer exposure times and the decreased turbulence levels (and therefore dispersion) found indoors. The paper discusses the role of building ventilation on the possible pathways of airborne particles and examine the fluid mechanics of the processes involved
Lessening the risk of transmission
At the current time there is no known medical cure against COVID-19, by vaccine or other physiological means. Therefore, the advice being given by PHE is to:
- lessen risk
- avoid inhaling/exhaling in any air which might happen to contain the virus by wearing a mask<
- follow government guidelines on social distancing and hand washing.
In addition, a range of engineered infection controls can be applied to reduce risks and these are described in this manual.
COVID is a pandemic
Whilst it would be ideal to prevent COVID-19 from entering a country, it has become clear that is very difficult to achieve, particularly in modern times with so much international travel for business or leisure.
The World Health Organisation (WHO) have been monitoring its spread across the world and have declared COVID-19 a pandemic because it is affecting many countries.
‘Worldometer’ is an organisation which presents data and is a provider of global COVID-19 statistics for many caring people around the world. Worldomoter data is trusted and used by many organisations including:
UK Government | Johns Hopkins CSSE | Government of Thailand | Government of Pakistan | Government of Thailand | Government of Sri Lanka | Government of Vietnam | Financial Times | The New York Times | Business Insider | BBC.
COVID-19 figures are reported in terms of cases, deaths, critical cases, recovering cases etc in the Worldometer which is reset after midnight GMT+0.
Worldometer reports that COVID-19 has entered 215 countries and territories round the world and two international conveyances as of 26 October 2020.
The list of countries and territories and their continental regional classification is based on the United Nations Geoscheme. Sources are provided under "Latest Updates"
Worldometer's figures make it clear that COVID has managed to cross borders into almost all countries despite attempts to block it.
Research to identify routes of transmission have shown the relevant strategies to deal with surface and airborne.
Research has also studied the risks associated with each, such as University of Nebraska work on Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1, published in the New England journal of Medicine, April 2020. This paper gives time scales for survival of virus on different materials as well as in the air. It followed earlier work on viral transmission reported at Antiviral Research in May 2016, 129:21-38. doi: 10.1016/j.antiviral.2016.01.012. Epub 2016 Feb 9.