What can deliberately infecting healthy people tell us about infectious diseases? How is this useful for developing treatments, and how do we manage the risks?
- The Prime Minister has announced that the 1m plus rule on social distancing and the legal obligation to wear face coverings in certain settings in England will end on 19 July, although social distancing and face coverings may still be recommended in certain settings.
- SARS-CoV-2, the virus which causes COVID-19 spreads through three main routes: inhalation of droplets suspended in the air, direct deposition of droplets on mucous membranes, and the carriage of virus particles from contaminated surfaces to the mucous membranes.
- The relative importance of each route is still the subject of debate, but the evidence now suggests that surface transmission and direct droplet deposition are less significant, while the inhalation of droplets is more significant than thought at the beginning of the pandemic.
- The risk of SARS-CoV-2 transmission depends on many factors, of which the important ones include proximity between people, duration and frequency of contact, ventilation and community prevalence of infections. Crowded, poorly ventilated indoor spaces are the highest risk settings for transmission. In such settings, superspreading events, where one infected person infects a large number of people, can occur.
- The risk of transmission can be reduced through vaccination, isolation of cases, adequate ventilation of indoor spaces, social distancing, wearing of masks, and good respiratory hygiene.
- Vaccination reduces transmission by reducing the number of people who become infected following exposure and by reducing how much viral material an infected person sheds.
On 5 July 2021, the Prime Minister announced that the 1m plus rule on social distancing and the legal obligation to wear face coverings in England will end on 19 July 2021. Instead, the Government will rely on people exercising personal responsibility to reduce SARS-CoV-2 transmission. This article discusses the different mechanisms by which the novel SARS-CoV-2 coronavirus is transmitted from one person to another, what we know about the importance of different transmission routes, how the scientific thinking has changed, and how transmission can be reduced. For explanations of many of the terms used in this article see the Covid-19 glossary.
What are the different ways of transmission?
The novel SARS-CoV-2 virus mainly infects cells in the human respiratory tract and uses them to replicate itself. It spreads when viral particles from an infected person reach another person’s mucous membranes, a type of tissue which lines the respiratory tract and parts of the eyes, and infect cells there. When an infected person expels breath, such as when they cough, sneeze or talk, respiratory fluids containing virus are expelled as small droplets.
- Direct deposition of droplets
- Contact and surface transmission
Transmission by inhalation
A major transmission route is thought to be the inhalation of viral particles. Respiratory droplets can stay suspended in the air for some time. Suspended droplets are also called an aerosol. If such droplets are inhaled, they can infect respiratory cells. How long droplets stay suspended in the air depends not only on their size, but also the velocity at which they were expelled from the body and the surrounding air’s temperature, humidity, and velocity. Larger droplets tend to fall to the ground quickly, while smaller droplets stay suspended in the air for longer and can travel longer distances. While larger particles stay suspended for much shorter periods of time, they can still be inhaled when close to an infected person. Very small airborne droplets can travel much longer distances (virus particles have been detected more than 50 meters from an infected patient) and may pose an infection risk even after the infected person has left the room.
These characteristics mean that the risk of transmission through inhalation varies according to a number of factors. These are explored in more detail in the following sections, but they include the:
- duration of the contact. The longer the duration of time someone spends close to an infected person, the greater the chances of inhaling a viral particle.
- activity taking place. The number of droplets emitted is greater during certain activities, such as when talking loudly and singing.
- distance between people. The greater the distance the lower the risk.
- how infectious the person is. People tend to expel the most viral particles when they start to get symptoms or just before they become symptomatic.
- ventilation, which helps to disperse aerosols and reduce the accumulation of virus-containing droplets in the air. The risk of infection is therefore much lower outside and is increased in poorly ventilated and crowded spaces.
Direct deposition of droplets
Another major transmission route is thought to be the direct deposition of respiratory droplets on mucous membranes in the mouth or nose. Respiratory droplets often leave the body at high speeds, such as when a person coughs or sneezes, and can travel considerable distances through the air. If a person is infected with SARS-CoV-2, these droplets can contain infectious virus particles. If these virus-containing droplets reach the mucous membranes in the mouth, nose or eyes of another person, that person can become infected.
If a person is standing in close proximity to an infected person, it is more likely that a droplet will reach them and therefore they have a higher risk of getting infected than someone who is farther away. Similar to inhalation-based transmission, the risk also depends on the duration of contact and how many virus containing droplets are emitted by the infected person.
Contact and surface transmission
SARS-CoV-2 can also be transmitted through contact with contaminated surfaces. When respiratory fluids from an infected person’s nose or mouth are deposited on a surface such as a door handle or railing, this can act as a means of transporting virus particles from an infected person to an uninfected person. Surfaces can become contaminated when an infected person sneezes or coughs on them, or when the infected person touches their mouth or nose immediately before touching the surface. If someone else touches the contaminated surface and subsequently touches their mouth, nose, or eyes, they may become infected.
The risk of this occurring depends on many factors including the amount of virus on the surface, the material of the surface and the length of time since it was deposited. Furthermore, if virus is deposited on a surface that is frequently touched, more people are at risk of becoming infected. This type of transmission could also occur by direct contact with an infected person, for example during a handshake.
Other (potential) ways of transmission
The World Health Organization (WHO) has suggested other potential mechanisms of transmission including: bloodborne transmission; breastmilk; human-to-animal; animal-to-human; and faecal-oral transmission. Currently, except for human-to-animal transmission, there is no available evidence to suggest these pathways are taking place. The three routes discussed above are likely to be most important and are the major focus of the policy response.
What is their relative importance and how has the thinking changed?
Determining how much each transmission route contributes to Covid-19 infections is difficult, because there is often very little data available regarding where, when and how someone has become infected. Early in the pandemic, studies that showed the virus can survive on surfaces for hours were widely publicised. This raised concerns that surface transmission may be a major route through which SARS-CoV-2 spreads. Since then, however, there have been “no specific reports which have directly demonstrated [surface] transmission” according to the WHO. This does not mean that infections caused by surface transmissions cannot take place and it remains important to wash hands and keep surfaces clean, but they are much less likely than originally thought.
In contrast, inhalation of suspended droplets (aerosols) was not widely considered to be a major transmission route at the beginning of the pandemic. The thinking on this, however, has changed and it is now widely acknowledged that SARS-CoV-2 also spreads through inhaling aerosols.
There is still disagreement regarding exactly how important transmission by inhalation is. SAGE states that close-range transmission through droplet deposition and inhalation of aerosols is likely to be more important than long-range aerosol-based transmission but acknowledges that there is “not yet sufficient evidence to confidently separate out the relative importance of these routes”.
There is also ongoing discussion in the scientific and medical community about how the term “airborne transmission” should be used. Traditionally, scientists use the term “airborne transmission” exclusively for long-distance inhalation-based transmission through very small droplets of <5 micrometres (µm – the width of a human hair is about 75µm). If the droplet has a diameter >5µm, these scientists refer to this as “droplet transmission”. For example, the WHO acknowledges that “airborne transmission” is possible, but states that “droplet transmission” is more likely. The 5µm boundary in this definition has been the subject of debate and criticism because it is not evidence-based.
Many scientists now use “airborne transmission” as a synonym for all transmission by inhalation, regardless of droplet size. They tend to emphasise that SARS-CoV-2 is airborne and some think it is “exclusively airborne”. This definition may be closer to the common understanding of the word “airborne”.
Factors which affect SARS-CoV-2 transmission
There are a number of factors that affect the risk of transmission – most importantly ventilation, proximity, duration and frequency of contact, and community prevalence of infections.
Indoor vs. Outdoor
The type of setting has an impact on the risk of infection. Generally, crowded, poorly ventilated indoor spaces are higher risk. Virus-containing aerosols can accumulate in indoor spaces with little ventilation. This makes it more likely that they are inhaled by others in the room. Outdoor settings are generally much safer because they tend to be well ventilated and aerosols do not accumulate. However, where outdoor spaces are crowded, or in some way enclosed this can increase the risk of transmission.
Increasing the distance between an infected person and others can significantly reduce the risk of infection. Large respiratory droplets fall to the ground quickly and cannot reach people who are at a distance. This reduces both droplet deposition and short-range inhalation-based transmission. Long-range transmission through smaller droplets may, however, still occur. There is evidence showing that outbreaks have occurred in poorly ventilated spaces despite physical distancing.
Duration, frequency and type of contact
Another important factor is the duration, frequency and type of contact between people. It is generally believed that a longer contact time or a higher frequency of contact, both with the same person or different people, leads to a higher risk of infection. This is especially true indoors, where the air can quickly become saturated with viral particles the longer an infected person stays. Some types of contact also carry a higher risk. An indoor exercise class has a higher transmission risk than a taxi ride, even though both are in enclosed spaces, because of activity involved.
Finally, the prevalence of SARS-CoV-2 infections in the community also impacts the absolute risk of infection. In areas with very low prevalence, the risk of being infected is lower across all settings. However, the risk indoors will still remain higher than outdoors.
What do these factors mean in terms of higher risk activities?
Given the factors influencing transmission outlined above, there are a number of settings which should be considered high risk for spreading SARS-CoV-2. In general, risk is highest in poorly ventilated, crowded indoor settings where people aren’t socially distanced, don’t wear face coverings and/or engage in activities that result in increased droplet/aerosol creation, notably singing, shouting, or aerobic exercise. Examples include bars, nightclubs, parties and social gatherings, indoor dining, gyms and exercise classes, choirs and churches.
In high risk settings, such as those listed above, superspreading events may occur. These are events where many people are infected by a single infected person.
In line with the factors affecting SARS-CoV-2 transmission outlined above, superspreading events often occur when large numbers of people meet in crowded, poorly ventilated indoor spaces, although other factors can also contribute.
On average, people infected with the original SARS-CoV-2 virus infect an estimated 2–4 further people (the R number), if no measures to reduce infections (e.g. social distancing) are taken and no one has immunity via prior infection or vaccination. However, because people spend different amounts of time in high-risk settings, some infected people do not spread the virus at all while others have been found to spread the virus to a much larger number of other people. It was estimated that fewer than 20% of infected people go on to infect 80% of secondary cases.
How can the risk of each type of transmission be reduced?
As the scientific understanding of SARS-CoV-2 transmission evolves, the guidance on how to best prevent infection has changed. Early in the pandemic, public health messaging focused on washing hands and disinfecting surfaces, while mask wearing was considered to be less important. Given the current understanding that aerosols do play an important part in transmission, the public health guidance has been amended to include the importance of wearing a face covering in indoor spaces and of fresh air. For example, the “hands – face – space” messaging has been updated to “hands – face – space – fresh air”.
Isolation of cases and contacts
An important way of reducing the risk of SARS-CoV-2 transmission is the isolation of infected people and their contacts. Current Government guidance is that that anyone who has: Covid-19 symptoms; has received a positive Covid-19 test; or lives in the same household as someone who has symptoms or tested positive should self-isolate for 10 days. By reducing contact with other people as much as possible, self-isolation reduces the risk of transmission through all of the three main transmission mechanisms.
The 1–2m social distancing guidance is largely based on direct droplet-based transmission through larger droplets. Even though direct deposition of droplets may not be the main route of transmission, social distancing remains very important. This is because inhalation-based transmission is likely dominated by short range aerosols of larger size that are still reduced by social distancing measures. Given that aerosols are also an important route of transmission, some experts think that more nuanced guidance may be more useful than simple distance cut offs. Such rules should take into account the setting in which a contact takes place. In crowded, poorly ventilated indoor spaces, keeping 2m distances may not be enough to adequately reduce infection risk. Similarly, settings with a lot of shouting or singing should be considered higher risk and more distance is needed.
The Government recently conducted a review on social distancing. The Prime Minister’s 5 July 2021 statement outlined that the 1m plus social distancing rule will no longer be in place from 19 July 2021.
One intervention that may prevent both droplet deposition and inhalation-based transmission of SARS-CoV-2 is wearing face coverings. A face covering is a piece of material, e.g. cloth, that covers someone’s nose and mouth. It does not need to be a medical grade face mask such as a surgical mask, although coverings with multiple layers may be more effective. Face coverings, when worn by both infected and uninfected persons, may reduce transmission by: reducing the number of droplets the infected person expels into the surrounding air; and reducing the risk of virus-containing droplets reaching the mucous membranes of an uninfected person.
The NHS currently advises wearing face coverings to reduce the risk of transmission, especially where it is difficult to keep distanced from other people. From 19 July there will no longer be a legal requirement to wear face coverings in certain settings, but the Government “expects and recommends that people continue to wear face coverings in crowded, enclosed spaces”.
The recognition of inhalation-based transmission as a major transmission route has led to a larger focus on adequate building ventilation as a measure for preventing infections. By improving ventilation, the air within a room is replaced more quickly and the risk of virus-containing aerosols building up is lower. Ventilation can be improved by: opening windows, especially at opposite sides of a room or home to create a draft; or mechanical ventilation systems which bring in fresh air from outside. The Health and Safety Executive (HSE) has issued guidance on ventilation in workplaces during the pandemic. HSE also recommends using carbon dioxide monitors to identify poorly ventilated areas.
Hand washing and cough hygiene
While the risk of surface transmission is not as high as previously thought, regular hand washing remains important. Regularly washing hands with soap can kill viruses and reduce the risk virus being deposited on surfaces or a person’s mucous membranes. Good coughing hygiene, i.e. coughing into a tissue or your elbow can reduce the number of droplets in the air and thereby reduces the risk of both inhalation and droplet-based transmission.
Perspex screens were adopted early in the pandemic as a way of preventing transmissions, but their effectiveness has since been called into question. They may help to reduce direct droplet-based transmission by preventing droplets from reaching another person’s mucous membranes and/or surface transmission by reducing contact with shared surfaces. They may not prevent inhalation-based transmission as particles suspended in the air can move around the screens. They could also increase the risk of airborne transmission in certain scenarios if screens interfere with ventilation. Given the emerging evidence for inhalation-based transmission, it may therefore be particularly important to ensure screens do not reduce ventilation in indoor settings.
Effect of vaccination on transmission
Vaccination can reduce transmission in two main ways: by reducing the number of people who become infected following exposure; and by reducing how much infectious viral material an infected person sheds. Current evidence suggests that all vaccines authorised for use in the UK are highly effective at protecting against symptomatic Covid-19. Emerging evidence also suggests that vaccines reduce viral load, thereby reducing how infectious (a vaccinated) infected person is. For a much more detailed discussion see our rapid response on vaccines and virus transmission.
New variants and transmission
During normal viral replication, errors in copying the virus’ genetic code, its RNA, can take place, which result in mutated forms (or variants) of the virus emerging.
While the underlying mechanisms of transmission outlined above remain the same, some variants of SARS-CoV-2 may have higher transmissibility than the original virus, also called wild type, which first originated in Wuhan, China in late 2019.
An example of this is the Alpha variant (B.1.1.7), first discovered in the UK. There is a lot of uncertainty regarding the main drivers of this increase, but the most likely explanation is that less virus is needed to infect another person (lower infectious dose) and/or an increase in viral shedding from an infected person.
In other variants, mutations might lead to a partial immune escape. Such a variant can, at least partially, circumvent the immunity acquired by natural infection or through a vaccine. People who have previously been infected with SARS-CoV-2 or vaccinated could be infected again with an immune escape variant. This may lead to higher transmission, because the population susceptible to the variant is greater than that of the original virus.
One such variant is the Delta variant (B.1.617.2), which was first discovered in India. Preliminary data from a PHE-funded study found “only modest differences in vaccine effectiveness with the [Delta] variant” after two doses of the Pfizer or AstraZeneca vaccines, meaning that the degree of immune escape is small for fully vaccinated individuals. The study also found that the degree of immune escape is larger for people who have only had their first dose. This means that partially vaccinated individuals have a higher risk of becoming infected with the Delta variant compared to the original virus or Alpha variant.
While the scientific understanding of SARS-CoV-2 transmission has grown massively since the beginning of the pandemic, there are many questions that remain unanswered. The exact relative importance of different transmission routes remains the subject of debate. So does the long-term effectiveness of vaccines, the effect of new variants on transmission and vaccine effectiveness, and to which extent booster jabs can offset any loss of vaccine effectiveness.
Answering these and other questions will be important for determining which measures for preventing SARS-CoV-2 transmission are most effective and may be needed in the future. It might also help to prevent future pandemics: some scientists have argued that a long-term shift in public health focus is needed to include airborne pathogens and address them in a similar way to waterborne and foodborne diseases.
POST would like to thank Professor Christl Donnelly (University of Oxford; Imperial College London) and Dr Natsuko Imai (Imperial College London) who acted as external peer reviewers in preparation of this article.
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