Representational image. Shoppers maintain social distance while standing in a queue at Lake Mall, Kolkata after it reopened Monday. | PTI
Representational image. Shoppers maintain social distance while standing in a queue at Lake Mall, Kolkata after it reopened | PTI Photo
Text Size:

Bengaluru: The primary mode of Covid-19 transmission continues to remain large droplets even as aerosol and fomite transmission cannot be ruled out, the World Health Organization (WHO) said Thursday in its new scientific brief summarising everything known about the virus’ transmission.

The WHO issued the brief after over 200 experts wrote an open letter to the agency to revise its airborne transmission guidelines.

In the 10-page brief that explored all possible modes of transmission for SARS-CoV-2, the global health agency admitted that aerosol transmission cannot be ruled out, especially in crowded and poorly ventilated areas.

Aerosols are defined to be less than 5 micrometers or five-thousandth of a millimetre in size. While droplets can also be airborne, airborne transmission refers primarily to aerosol transmission in the context of Covid.

Aerosols can stay in the air for several hours and spread widely, especially in unventilated indoor spaces.

Scientists suspect that SARS-CoV-2 might be transmitted via aerosols, which are generated from just speech or exhalation of air. In their letter, the experts had described airborne or aerosol transmission as the cause behind some super-spreader events.

Also read: Why Delhi’s Covid hospitals are struggling to fill ‘lucrative’ senior resident doctor posts

‘Other modes of transmission can also explain some events’

The latest WHO document acknowledged experiments that demonstrate the release of aerosols during actions like talking or singing, but said that aerosol transmission has not been established or demonstrated with the SARS-CoV-2 virus.

It added that even though the virus was detected in the air after aerosol generating procedures in hospitals, it was found to not be viable.

On Tuesday, Benedetta Allegranzi, WHO’s technical lead for infection prevention and control, had addressed the concerns raised in the open letter, saying, “There is some evidence emerging, but it is not definitive”. “The possibility of airborne transmission in public settings, especially in very specific conditions — crowded, closed, poorly ventilated settings — cannot be ruled out,” she had said.

In the new brief, the agency said there have been some reports that seem to suggest aerosol transmission had spread the disease, but those can be explained by other modes of transmission of the virus, including “during choir practice, in restaurant or in fitness classes”.

There have been instances of super-spreader events in both a choir practice in Washington as well as in restaurants in China that have been linked to aerosol transmission.

“In these events, short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces, over a prolonged period of time with infected persons cannot be ruled out. However, the detailed investigations of these clusters suggest that droplet and fomite transmission could also explain human-to-human transmission within these clusters,” said the WHO.

Fomite transmission is when a healthy person contracts the virus from a contaminated surface.

The agency also explained that not taking adequate distancing or masking measures would have further facilitated such a spread.

In their letter, the scientists had compared the evidence the scientific community has for aerosol transmission with that for fomite transmission, alleging that WHO stresses more on fomites despite not having evidence that it’s a prime mode of transmission. They had said that there is “admittedly incomplete” evidence for microdroplet transmission, just as there is for both large droplet and fomite transmission.

The WHO brief acknowledged the lack of evidence for fomite transmission, stating that despite consistent evidence of the virus settling on surfaces, there has been no evidence of the virus spreading between individuals through this method, as those who came in contact with infected surfaces likely came in contact with infected individuals as well.


To prevent transmission, WHO continued to recommend testing, tracing, and isolating. It said asymptomatic people can infect others, and those displaying symptoms infect others primarily through droplets and close contact.

The agency also said that further research is urgently needed to investigate aerosol transmission understanding instances attributed to it.

Also read: Plasma therapy — 11 trials underway, many Covid ‘cure’ stories but some doubts linger


Subscribe to our channels on YouTube & Telegram

Why news media is in crisis & How you can fix it

India needs free, fair, non-hyphenated and questioning journalism even more as it faces multiple crises.

But the news media is in a crisis of its own. There have been brutal layoffs and pay-cuts. The best of journalism is shrinking, yielding to crude prime-time spectacle.

ThePrint has the finest young reporters, columnists and editors working for it. Sustaining journalism of this quality needs smart and thinking people like you to pay for it. Whether you live in India or overseas, you can do it here.

Support Our Journalism

1 Comment Share Your Views


  1. New theory emerging in early July 2020 that the novel Coronavirus ( Covid-19 ) is airborne.
    The world today is getting more and more infected by novel coronavirus officially given the name of Covid-19. It is spreading since it came to notice in January , 2020. The World Health Organisation whose duty it is to take steps to alert the global community about epidemics or pandemics and suggest all that is necessary to contain such deadly diseases. In this regard , the WHO has been until recently of the view that – “ the Covid-19 spreads primarily from person to person through small droplets from the nose and mouth which are expelled when a person with Covid-19 coughs , sneezes or speaks”. But something new related to this scientific phenomenon about Covid-19 has been expressed in early July 2020 by 239 scientists of 32 countries to show that the novel coronavirus spreads through air. In other words , they have opined that Covid-19 is airborne. The novel coronavirus in small particles in the air can infect people. The WHO has been urged to revise its recommendations in that regard. A detailed report with evidence is proposed by the scientists to be made available in days to come. It is being said that WHO seems to be impressed by the new theory but may need more going into before they can advise or recommend to the people. In this regard , basic question may have to be addressed with closer clarity for common man as to what difference the new theory makes to the earlier theory of WHO. The earlier theory was that the virus spreads through person to person through small droplets from the nose and mouth which are expelled when a person with Covid-19 coughs , sneezes or speaks. Obviously , the passage or medium is open space between one person and another. And there is limitation to space where small droplets can stay and how long. This was addressed by the WHO so far. It seemed the force or strength of sneezing or cough or speaking would bringing out the smaller particles to the space between the persons , transporting to the being by entering into nose or mouth. But the new theory looks like putting forth that the smaller particles of the coronavirus stay in the air and even if the person or persons emitting smaller particles have gone , the air possessing those smaller particles can enter through nose or mouth or respiratory system in the body to infect. But all these circumstances would need to be clarified as to the limitations of distance and time to the smaller particles to stay in air and travel through air to infect. Let us wait for more details and response of WHO as well. But if the new theory of how Covid-19 is airborne and can spread through air is accepted as a scientific fact , it is obviously a new or addition to worry for global community. However , that may suggest to the accuracy of something relevant opined by this Vedic astrology writer in the alert on 11 November , 2019 through predictive article – “ Astrological probable alerts for 2020” – published at on 1 January , 2020. The text of the opinion referred here reads like this in the predictive article of 11 November , 2019 :-
    “ 4. …………………………….. More care and appropriate strategy may be taken during April to June , particularly May-June in 2020 , against spill or loss of such stuff as are known for repugnant and repulsive smell though air passage”. The concept of airborne is clear here while other part is implied in the predictive alert , when applied to Covid-19.


Please enter your comment!
Please enter your name here