Explained: What does airborne transmission of COVID-19 mean?

After more than 240 scientists from 32 countries wrote an open letter to it warning about airborne transmission of COVID-19 via small microdroplets, the World Health Organisation (WHO) was forced to take notice. WHO had, until then, been downplaying airborne transmission of COVID-19 and had been instead advocating washing of hands regularly.

The organisation had so far maintained that the virus is transmitted through respiratory droplets when people speak, cough or sneeze. These droplets fall to the ground or objects or surfaces around the person. WHO had said that the virus primarily spreads through person-person contact.

But the scientific community is now saying that the virus is transmitted through air and can infect individuals when inhaled.

What does that mean? In case a person sneezes, the droplets can travel quickly through the length of the room beyond 2 metres. According to these scientists, at typical indoor air velocities, a 5-micron droplet will travel tens of meters, much greater than the scale of a typical room, while settling from a height of 1.5 m to the floor. The airborne transmission occurs in addition to the large droplet and surface (fomite) routes.

While hand washing and social distancing were appropriate, the scientists underlined that such measures were insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people.

These smaller droplets can be called aerosols (size of an aerosol ranges from 0.001 μm–10 μm) and since they are small in size, they can stay in the air for 10-15 minutes and can be moved around by gusts of winds, explained Dr Soumya Swaminathan, WHO’s chief scientist.

Several studies conducted after the SARS-CoV-1 epidemic demonstrated that airborne transmission was the most likely mechanism explaining the spatial pattern of infections, stated the scientists in their letter.

This would mean that masks would become important even indoors whether or not physical distancing has been maintained. Additionally, all healthcare workers would require N95 masks compulsorily to filter out the smallest droplets while taking care of infected patients.

This problem is especially acute in indoor or enclosed environments. Indoor spaces without enough ventilation and air filters such as factories, offices, schools, public transport, nursing homes, restaurants and religious spaces would be categorised as high-risk.

To mitigate the transmission indoors, general ventilation must be supplemented with airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights. Indoor cross-ventilation could help a number of these spaces.

WHO admitted that such transmission is possible, but evidence will have to be thoroughly evaluated. If it is confirmed, it would mean that the widespread use of masks has to become the norm especially in crowded spaces.