COVID-19 Manual Section 2: Transmission of COVID-19
Further reading
- European Centre for Disease Prevention and Control - an agency of the European Union has published guidance entitled ‘Transmission of COVID-19'.
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.
Airborne
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.
Surface contacts
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.
Airborne transmission
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.
Stage 1: At the source
Commencing at the Source (the face) COVID spread can be limited by simple measures of 'personal infection control' (PIC):
- Wearing a suitable mask – see Section 5 Transmissions and masks – can limit spread of droplets and aerosols
- Maintaining ‘social distances’ between people, which means keeping more than 2m apart
- Washing /disinfecting hands to limit spread of COVID particles on surfaces.
The UK Government website provides detailed advice and summarises this approach as 'Wash hands, cover face, make space'. Isolating when sick, and quarantining when you have been exposed is also an essential part of source control, preventing others from being exposed to the virus, especially as people are often infectious before they have symptoms.
Stage 2: In high risk areas
Where there is a risk of meeting a person with COVID such as:
- Restaurants and bars
- theatres
- sports centres
- shops and supermarkets
- transport
- care homes
- workplaces
using infection control and risk management techniques and equipment, implemented/installed with the assistance of engineers.
These range from barriers, screens and air cleaners such as UV sterilisers and HEPA filters. Effective ventilation is an essential part of this engineering solution.
Stage 3: In communities
- lockdowns
- restriction of certain activities
- limitations on numbers of people in spaces,
which impose restrictions on movement and activities. Such measures impose limitations on the normal way of life and impact on businesses, so are only used when necessary.
Stage 4: Following infection through Track-and-Trace
Either backtracking to find its source or forward to wherever it might be spreading next. Modern highly automated systems using tracking through data sources, for example mobile phones, can identify whenever and where a COVID-19 carrier met others and for how long.
This enables a way to catch and stop a COVID spread in its tracks. This is supported by good record keeping and policies in organisations to enable contact tracing teams to understand when and where people may have been exposed.
Stage 5: Design for living with COVID-19
COVID has exposed weaknesses in our modern way of life toward contagious pathogens. We live with an assumption that the world is clean and if an infection occurs we can rely on antibiotics or other medication to deal with it. However COVID has reminded us that nature is continually developing new strains and in the case of COVID they can be very infectious and harmful – and we must develop new cures!
We can and should design to live with such risks, for example, using card payments or using proximity readers rather than cash potentially reduces transmission of pathogens on shared surfaces. However, there are many and much larger engineering solutions, which may require significant capital investment.
In construction, we can build with safety and health in mind. Chicago's Fulton East Tower is billed as one of the nation's first post-COVID-19 structures designed with enhanced air filtration, widely-spaced offices, and other touchless features.
Within buildings we can apply engineering to achieve safe and healthy occupancy using techniques such as air disinfection. Equipment such as UV air sterilisation is moving from occasional use to widespread application.
Although many of these solutions represent a significant investment, they also represent an opportunity to stimulate innovation to tackle a range of issues including climate change, energy use and poor air quality alongside designing for infectious disease control. This was discussed by several members of the institution at a media briefing on engineering controls in June 2020