How is COVID-19 spread? Here’s what scientists know (so far)


There is not yet a foolproof scientific consensus on how the coronavirus that causes COVID-19 spreads, but experts have a pretty good idea. This understanding must be at the heart of our strategy to fight the virus with minimal collateral damage.
It is not an over-reaction or under-reaction, but a bad reaction. For example, sending janitors to rush to closed schools and other buildings to disinfect them poses far more risk to these janitors – and anyone they come in contact with later – than leaving the buildings empty.

And if you leave your building to run, there is more danger in your common elevator than in large spaces.

These distinctions are important. There is ample evidence that we are going to face this thing not for days or weeks, but for months.

Shelter in place directives are more suited to short-term emergencies such as terrorist attacks than to ongoing pandemics. It is simply not possible to tell people to stay at home for months. Many people will have to leave their homes to provide medical care, buy food and take out the garbage.

As we settle in for the long term, science-based public messaging should separate high-risk from low-risk or minimal-risk activities. It is essential that policies do not limit important types of work that pose almost no risk, or ignore risky activities that have fallen through the cracks.

These conclusions are based on what we know about how this virus spreads: people infected with the virus can expel infectious virus particles when they sneeze or cough, and also when they speak. Scientists have estimated that humans incubate the coronavirus for an average of five days, but sometimes up to two weeks before they have visible symptoms. There is evidence, recently published in Science, that pre-symptomatic people or those with mild symptoms are behind much of the spread.

But there are also many things that we don’t know. We do not know if some people, especially children, have completely silent infections. And there is still uncertainty about the size of the virus-carrying particles.

Viruses are just bands of genetic material surrounded by certain proteins; they can travel alone, but more likely on droplets of fluid of different sizes. Larger droplets settle faster and are more likely to spread the disease through close contact, especially direct contact such as shaking hands or touching common surfaces. Small aerosol particles can stay in the air longer, where others can inhale them. Airborne particles are even smaller – viruses are not attached to water and can therefore remain suspended in the air for hours. Measles and chickenpox are spread this way, which is why they are so notoriously contagious.

Members of Joint Task Force 2, made up of soldiers and airmen from the New York Army and the Air National Guard, work to disinfect New Rochelle High School during the COVID-19 epidemic in New Rochelle, New York, March 21, 2020.

Andrew Kelly / Reuters

Although the spread of COVID-19 aerosols is terrifying, several researchers I contacted over the weekend said that the data we suggest so far is unlikely – droplets appear to be the main form of contagion. However, when I spoke to Jeffrey Shaman of Columbia University, who is studying how the environment affects infectious diseases, he said that we do not yet know what types of particles are released – droplets, aerosols or suspended in the air. Harvey Rubin, of Penn, who studies complex biomolecular systems, says these categories are susceptible to mixing – the virus can move mainly on droplets, but there may still be some that float around particles of smaller or even lighter aerosols suspended in the air.

So, if there is currently no evidence of airborne transmission, but it cannot be excluded, should we assume that this is happening? It is important to remember that a person must meet a certain critical mass of virus to be infected. This means that even if a few particles are transmitted into the air, they may not be the cause of the disease in the real world.

A study recently published in the New England Journal of Medicine on viruses surviving for months on surfaces should be viewed in the same probabilistic light. Scientists were able to detect small amounts of the virus on plastic and stainless steel several days after they were deposited. But Purdue University virologist David Sanders says it shouldn’t be taken to mean that a surface touched three days ago is as dangerous as an infected surface just touched. The amount of virus is decreasing approximately exponentially, which is why it may not be rational or human to send janitors into just closed buildings.

A woman wearing a face mask as a preventive measure against the spread of COVID-19 pushes a shopping cart.

Guillermo Arias / AFP via Getty Images

Part of the science that was missed in the post is about confined spaces – such as elevators in apartment buildings. However, if there is one place where you could inhale persistent particles, it would be in the elevator of an overcrowded building. Therefore, it would be better to take the stairs, if you can – they would provide young people and able-bodied people with essential exercises and reduce the risk of taking the elevator for elderly or disabled residents.

If some smaller particles hang in the air, they would be dispersed and diluted almost instantly outside. This is why most of the experts I spoke with said that despite orders to stay at home, they walked, ran or cycled. It is not a compromise between public health and personal good, because you do not put yourself or others in danger unless you approach others.

We need to keep an eye on the activities most likely to spread this virus. As the Washington Post reported on a CNN open chat program, even Tony Fauci warns of the obsession with a single virus particle that could cling to cardboard packaging: “I think if you start thinking about money and mail and things like that, you can almost sort of stand still, which in my opinion is not a good idea. ”

There are many things we do not yet know about COVID-19. But we can better use the facts we have to shape our response.

This column does not necessarily reflect the opinion of Bloomberg LP and its owners.

Faye Flam is a columnist at Bloomberg Opinion. She has written for The Economist, The New York Times, The Washington Post, Psychology Today, Science and other publications. She graduated in geophysics from the California Institute of Technology.


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