About a year ago the first case of COVID-19 was reported in India. In the past year a lot of research has been conducted on the nature of the virus and new studies are being published even now. In this time, the mode of transmission of the virus was considered to be through fomites, or through surface transmission. As more research is done, we see that the chances of transmission through fomites, while theoretically valid, is still minimal when compared to aerosol or droplet transmission.
Let’s jump into the basics of these.
What is fomite transmission?
Say a person sneezes in your vicinity. Droplets are expelled from the person’s mouth.
Heavier droplets fall closer to the person who expels them, increasing risk of infection at close contact. When these droplets fall on surfaces, they become fomites.
Surface or fomite transmission occurs when a virus with the capacity to infect is present on a surface that is touched, and then the hand touches the nose, mouth or eyes. Precautionary measures to prevent this are disinfecting surfaces and washing hands.
Early last year, in a mixed-use building in South Korea, workers in a call centre became ill with Covid-19, infecting many others in that floor of the office. But in the rest of the occupants of that building (almost 1,000 people), only three other people became positive for Covid-19, which could have been through being in close contact with infected patients for extended time periods or aerosols.
It has been argued that had there been significant fomite transmission, many more people in the building would have tested positive, given that there were other shared spaces like elevators and lobbies.
What is aerosol transmission?
Aerosols can be exhaled when simply breathing, when coughing or sneezing, even speaking and singing. Expulsions during coughing, singing or even talking loudly reportedly increase the distance that these aerosols travel.
There have been other singing events, as well as other clusters of superspreading events that have been linked to transmission indoors and in enclosed spaces like prisons and hospitals, attributed primarily to aerosol spread, driven by air currents.
In an air-conditioned restaurant in Guangzhou, China, a five-member family, including an asymptomatic patient, ate on 24 January 2020. Within two weeks, nine others who had dined there the same day fell sick. Four were relatives of the index patient but five others had caught the virus while seated on two different tables with others who did not get infected.
So, what is the difference?
In the airborne terminology, the sizes of particles differ.
Humans produce respiratory droplets that range from 0.1 micrometre to 1,000 micrometres in size. Larger droplets can be anywhere between 5 and 1,000 micrometres in size, while what is generally known as aerosols are less than 5 micrometres. The SARS-CoV-2 virus is approximately 0.1 micrometre in size.
Expelled respiratory droplets that are heavier and immediately fall to the ground within a distance of 1-2 meters of the source increase the risk of infection at close contact.
Airborne transmission occurs when suspended respiratory particles, like cough droplets, are dispersed across the area by air currents.
Aerosol transmission of a virus typically involves the virus either floating free in the air, or latching on to other aerosols — dust or pollutants, but mainly phlegm or water, which can encase the virus particles before evaporating away — and floating to larger distances.
Several case studies have been published that highlight much higher transmission of COVID-19 through aerosols than compared to that through fomites.
This is also the reason why in June 2020, a study published in the peer-reviewed Environmental Protection concluded that people in polluted areas are more severely affected by Covid-19.
So what do we do?
While there is no harm in sanitising surfaces and keeping your hands clean, it is important to wear masks, preferably triple layered ones and avoiding packed indoor spaces that aren’t well ventilated. The absence of ventilation causes the particles to stay suspended in the air for longer, thereby increasing chances of infection.