Last week airborne transmission became a major problem in the public debate on coronavirus (COVID-19). More than 200 scientists around the world have written a letter to the World Health Organization (WHO) urging it to take seriously the growing amount of evidence that coronavirus can be transmitted through the air. Although WHO has not yet redefined SARS-CoV-2 (the virus that causes COVID-19) as air-transmittable, but it does recognize that “more urgent research is needed on the subject and analyses of its importance in COVID-19 transmission”.
Chad Roy, a microbiologist at Tulane University in the United States, explains: “Honestly, I don’t know what people expect. WHO does not need to go out and proclaim airborne transmission for us to recognize that this is an airborne disease. I don’t know how much clearer the scientific evidence must be.”
But what does “air transmission” mean in this context? Basically it is a matter of size. It is quite clear that SARS-CoV-2 is transmitted through tiny droplets that contain viral particles capable of causing infection. However, an airborne virus is defined with differences, depending on the expert who is asked. In general, this definition indicates that it can be spread by inhaling small particles known as aerosols over long distances, perhaps even from separate rooms.
“That’s why when some professionals are asked if the virus is air-transmitted, they will say no, because we are not detecting transmission to that type of distance,” explains the retired public health teacher who currently works as a consultant for companies and organizations Lisa Brosseau.
There is also some debate about what we mean by “aerosols“. Droplets that carry viral particles through the air can be of different sizes, but while larger droplets quickly fall to the ground or other surfaces, smaller ones (only a few microns wide) can remain suspended in the air for a while, allowing them to be inhaled. The word aerosol is mainly used to describe these smaller particles, although Brosseau would prefer the term “aerosol transmission” to cover the full range of inhalable viral particles that are expelled into the air, both large and small.
If SARS-CoV-2 is transmitted through the air, it is not the only disease of this type. Measles is known to stay in the air for up to two hours. Tuberculosis, although a bacterium, can be transmitted through the air for six hours, and Brosseau suggests that coronavirus super propagators (people who appear to expel more viruses than others) spread the virus in patterns reminiscent of the type of TB infection.
It could be said that there is already evidence that this type of transmission with SARS-CoV-2. Several large studies point to airborne transmission as an important route of spread of the virus’s COVID-19. Other work suggests that it can remain in aerosol droplets for hours. A new study led by Roy and his team shows that infectious aerosol particles with SARS-CoV-2 could actually be airborne for up to 16 hours and maintain their infectious capacity much longer than MERS and SARS-CoV- 1 (other important coronaviruses of this century).
We still don’t know what gives SARS-CoV-2 the airborne characteristic. “But it may be one of the reasons why it is a pandemic and not just a small outbreak as with any other coronavirus,” Roy believes.
Change of strategy
Whether the virus is transmitted through the air is not simply a scientific question. If confirmed, it could mean that in places where the virus has not been adequately contained (for example, in the US), the economy needs to re-activate more slowly, under stricter regulations that reinforce current health practices, and with the introduction of others improved. Our current tactics to stop the spread will not be enough.
Roy would like to see more vigorous regulations on the strict use of face masks for everyone who leaves the house. The expert explains: “This virus spreads like crazy. Wearing masks can help a lot to stop transmission. I think that anything that promotes the use of masks would be useful to stop the production of aerosols in the environment.”
However, Brosseau stresses that while masks can limit the spread of large particles, they do not help much with smaller ones, especially if they are worn very loose. And he adds: “I would like us to stop trusting the idea that covering our faces will solve everything and that will help to flatten the curve. It is a magical thought, it will not happen.” For masks to really make a difference, they should be used all the time, even as a family.
It considers the evidence to lead to the conclusion that airborne transmission is “the primary and arguably the most important mode of spread for SARS-CoV-2.” He adds, “I think the amount of time and effort spent disinfecting each surface over and over again has been a great waste of time. We don’t need to worry so much about cleaning every surface we touch.” Instead, the focus should be on other factors, such as where we spend our time.
One of the biggest questions we still have about COVID-19 is how much viral load is needed to cause the infection. The answer changes if the concern is in the aerosols. Smaller particles do not carry as much viral load as larger ones, but since they can remain suspended in the air for much longer, it may not matter because they will accumulate in larger concentrations and will be more widely distributed more widely the longer an infected person is out there expelling the virus in the form of aerosols.
The more people enter and leave an enclosed space, the more likely an infected person will appear. The more time the infected people spend in that space, the greater the concentration of viruses in the air over time. This is bad news, especially for spaces where people congregate for hours, such as restaurants, bars, offices, classrooms, and churches.
The airborne transmission does not necessarily mean that these locations should remain closed (although that would be ideal). But cleaning surfaces with disinfectant and forcing everyone to wear masks won’t be enough. To stay open safely, not only will they have to reduce their capacity, but they will also have to reduce the amount of time people can spend there. Increasing social distancing beyond one and a half meters would also help keep people safer.
Furthermore, ventilation should become a top priority. This is going to be a big problem for older buildings that tend to have worse ventilation systems, and areas with many of those systems may have to stay closed for considerably longer. The impact of asymptomatic spread (transmission by people who don’t feel sick) and super- propagators only further compounds this problem. But research by the U.S. Department of Homeland Security shows that, in the presence of ultraviolet (UV) light, the particles in aerosols the size studied by Tulane researchers would disappear in less than a minute. Several companies have started using robots with UV rays to disinfect hospital rooms, shopping malls, shops, public transport stations, etc.
For many places, the biggest delays in economic reopening could ultimately be the price you pay to control the virus. Otherwise, the event that occurred when the only bar opened in Michigan,USA caused an outbreak of more than 170 new cases could become commonplace.
For many places, the biggest delays in reopening the economy could ultimately be the price to pay to control the virus. Otherwise, the event that occurred when the only open bar in Michigan (USA) caused an outbreak of more than 170 new cases could become common.
Looking ahead to fall, “the implications are profound, but not so difficult to understand,” says aerobiology expert at the University of Maryland (USA) and one of the authors who signed the letter to the WHO, Donald Milton. The researcher details: “We must give subsidies to bars and restaurants so that they remain closed. We have to increase ventilation where we can and begin to make the most extensive use of germicidal UV air sanitation in the upper part of the rooms and such Even more UV in places that should remain open, such as elementary schools. We have to space the hours of entry to work and keep the density low on public transport, or open windows. And we must wear masks.”