COVID-19 Manual Section 2: Transmission of COVID-19

Introduction

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.

Further reading

  • European Centre for Disease Prevention and Control - an agency of the European Union has published guidance entitled ‘Transmission of COVID-19'.

Epidemiology of COVID-19

When a new infectious disease is discovered, scientists called epidemiologists work with other scientists to find:

  • who has it
  • why they have it
  • and what can be done about it
    • how to protect against infections
    • how to supress it before it spreads.

Sources include the US Center for Disease Control CDC and the UK Government website.

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.

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.

Mitigating measures with application of engineering

Meeting report: 4th ISIRV antiviral group conference: Novel antiviral therapies for influenza and other respiratory viruses, Jennifer L McKimm-BreschkinAlicia M Fry.

An engineered approach to COVID risk reduction should identify stages where COVID can be stopped or reduced.

Five stages of infection control

Once COVID-19 gets into a country or a community in a country, it is necessary to take steps to limit its spread. This generally means preventing it spreading from one infected person to others.

The general principle is that COVID-19 ‘walks’ into a community within its host person. Control measures must focus onto the interactions between the host and others.

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

    By more costly and widespread public health measures such as:

    • 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

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