How does COVID-19 spread?

What are the different ways that people can spread the virus? 

 

There are three main ways that the virus can be spread between people: fomite transmission, droplet transmission and aerosol transmission.


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Fomite transmission

Surfaces that are likely to carry infection are called fomites. A surface may become contaminated by a person who has Covid-19, and then another person might touch that surface and then their mouth, nose or eyes.


Current evidence suggests that this is unlikely to be the main way that Covid-19 is spread.

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Droplet transmission

When a person breathes, talks, coughs or sneezes, they expel fluid droplets (containing water, mucus, and disease particles). If a person is infected, these droplets will contain viral particles. If these droplets come into contact with your mouth, nose or eyes, you can become infected. 

 

Current evidence suggests that this is likely to be the main way that Covid-19 is spread.

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Aerosol transmission

Aerosols are very small droplets of fluid (less than 5μm in diameter) and are produced when we breathe, speak, cough or sneeze. 

 

Evidence is changing around aerosol transmission. Up until now, most experiments failed to get active viral particles from the air, but a recent experiment has succeeded, showing that aerosol transmission may be another way that the virus can spread. 

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Depending on the type of surface, humidity, temperature several studies have shown that fomites can have viable virus particles present after a period of hours to days.3-4 When a person comes into contact with the virus indirectly, through touching a contaminated surface and then their mouth, nose or eyes transmission may be possible.

 

The good news is that hand hygiene practices and cleaning regularly used surfaces reduce the likelihood of transmission (soap is very effective at deactivating Covid-19).

Respiratory viruses are generally understood to spread via fluid droplets that one person expels, that can then be breathed in or make contact with another person. 5

 

The exact size of what constitutes a “droplet” rather than an “aerosol” is disputed 6, but the way that droplets and aerosols are differentiated in the scientific literature is that droplets are heavy enough to fall out of the air before they travel very far (1-2m), whereas aerosols stay in the air for longer (potentially forever if there is no airflow in an enclosed space).

Current government guidance around social distancing is based on the idea that most large droplets should fall out of the air within 1-2m of the contagious individual. 7 

 

However, we know that in reality other factors, including good ventilation, mask-use, and whether a person is breathing heavily, affect the likelihood of disease transmission - not just social distancing. 8

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Growing evidence suggests that aerosols play a part in passing on COVID-19. 9

 

Laser experiments have shown how aerosols are produced when we speak. 10 This means that if SARS-CoV-2 is able to spread through aerosols, the risk of passing on the virus is much greater than we thought. 

 

Recent studies have proven just that. Half life experiments have found that the virus can remain viable and infectious in aerosols for hours and on surfaces for up to days. 11

Previously research carried out in the real world has struggled to show that COVID-19 encased in aerosol droplets could be viable, partially because it’s really difficult to pull a virus from the air, isolate it, and then artificially infect cells with it without damaging it. A team in Florida recently used a new technique to catch the virus in the act at a real-life hospital, showing that aerosol transmission is possible.

 

The Florida team isolated viable concentrations of SARS-CoV-2 from air samples taken 2 - 4.8 meters from an infected person in a hospital room, further than the distance you would expect from droplet transmission (which is less than 2 meters). The isolated sample was confirmed to be genetically identical to cultures taken from the infected patient. 12

 

This study is waiting on peer review, but it’s pre-publication shows that we should be seriously considering the implications of airborne transmission. 

 

There is enough evidence to support precautionary measures against airborne transmission including;

 

  • Sufficient ventilation in indoor spaces

  • Airborne infection controls including air filtration

  • Avoiding overcrowding

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