COVID-19 airborne transmission possible in some indoor settings: WHO

Asymptomatic patients can pass on virus, extent unknown

By Banjot Kaur
Published: Friday 10 July 2020
The airborne transmission of COVID-19 in closed and improperly ventilated indoor settings is possible, says WHO. Photo:

The World Health Organization (WHO) has said airborne transmission of the novel coronavirus (SARS-CoV-2) in closed and improperly ventilated indoor settings is possible.

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, the UN health body said in its scientific brief issued on July 9, 2020.

It cited examples of choir practice places, restaurants or fitness classes as some of these settings.

“The detailed investigations of these clusters suggest that droplet and fomite transmission could also explain human-to-human transmission within these clusters. Further, the close contact environments of these clusters may have facilitated transmission from a small number of cases to many other people (for example superspreading event), especially if hand hygiene was not performed and masks were not used.”

Aerosol transmission

Before this scientific brief, the WHO’s categorical position had been that the virus did not float in the air, except in certain hospital settings. In a fact-check posted on social media on March 29, it had said the novel coronavirus disease (COVID-19) was not an airborne disease and could only spread through respiratory droplets that could travel a distance of only one metre.

The virus transmission was happening through droplets that were heavy and therefore quickly fell on the floor, without floating in the atmosphere, the WHO had said.

But now, the revised brief say that in the specific indoor settings described above, droplets that are smaller in size (less than 5 micrometres) and are less heavy, can actually remain in the air and travel over long distances and time periods. This is called aerosol or airborne transmission.

The WHO had acknowledged the evidence presented by 241 researchers in its July 8, 2020, press conference, which said it was time to acknowledge the airborne transmission of the novel coronavirus disease. At that time, the UN health body had said it would look into the evidence.

The WHO clarified:

“Another recent experimental model found that healthy individuals can produce aerosols through coughing and talking. Another model suggested high variability between individuals in terms of particle emission rates during speech with increased rates correlated with increased amplitude of vocalisation. To date, transmission of SARS-CoV-2 by this type of aerosol route has not been demonstrated. Much more research is needed, given the possible implications of such route of transmission.”

Hospital settings

The WHO said if a hospital was treating COVID-19 patients and was performing medical procedures that did not generate aerosols, healthcare workers could be infected, according to some studies. However, some other studies suggested they could not be, it added.

“Further studies are needed to determine whether it is possible to detect viable SARS-CoV-2 in air samples from settings where no procedures that generate aerosols are performed and what role aerosols might play in transmission,” it said.

However, in medical settings where procedures that generated aerosols were performed, airborne transmission could occur, it added.

Asymptomatic transmission

The UN health body has also made quite a few flip-flops about whether those who have no COVID-19 symptoms, but are found to be positive for SARS-CoV-2, are able to transmit the disease or not.

In a press conference on June 8, the WHO said the possibility of virus transmission from asymptomatic cases was “very rare.”

On June 9, the WHO held a special session to clarify its stand after it drew sharp criticism from experts on this. In the clarification, it said, “We do not know answers yet.”

In July 9’s brief, however, the WHO explicitly said that asymptomatic COVID-19 patients could spread the virus too. It qualified that while asymptomatic patients could pass the virus to others, the extent of this occurrence was not clear yet and more research was needed in this area.

“Information from contact tracing efforts reported to WHO by member states, available transmission studies and a recent pre-print systematic review suggest that individuals without symptoms are less likely to transmit the virus than those who develop symptoms,” the brief said.

“Four individual studies from Brunei, China, Taiwan and the Republic of Korea found that between 0 per cent and 2.2 per cent of people with asymptomatic infection infected anyone else, compared to 0.8-15.4 per cent of people with symptoms,” it said.

The use of masks, hand hygiene and social distancing remain the WHO’s key advice to people.

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