On Friday the CDC posted updated guidelines on the spread of SARS-CoV-2, the virus that causes COVID-19. They have been updating their website regularly, as new information becomes available, so this was not unusual. But it was noticed that the new recommendations include some significant changes, specifically the CDC warned about airborne transmission of the virus, something previously considered to be a minor risk. However, they quickly reversed course and pulled the new recommendations, claiming that the update was just a draft and the publication a mistake. The site now has reverted to their prior warnings, focusing on large droplet spread.

All of this would be of very little interest (mistakes like this are not uncommon or particularly suspicious), except there already is a grave concern of political interference at the CDC. Politico, for example, recently reported that White House officials have been regularly reviewing CDC reports, and changing wording to align the reports more with Trump’s political interests. They have also been trying to shut down reporting from the CDC altogether. In this light, the recent SNAFU by the CDC can seem more nefarious. At the very least they no longer deserve the benefit of the doubt.

Let’s put the recent updating-then-backtracking on transmission guidelines into perspective – how is COVID-19 spread?

There is a strong consensus, and no real doubt, that the primary way in which COVID-19 is spread is through large droplets. There are droplets that come from breathing, talking, coughing, sneezing, shouting, or singing. Viral particles are encapsulated in globs of mucus, saliva, and water. These globs can spread several feet, exactly how far is still a little controversial, but 6 feet is generally considered to be a minimal safe distance.

Mechanism of spread is tied to the methods that will prevent spread. Therefore, social distancing – keeping at least six feet away from other people – reduces the risk of being sprayed by infected droplets. Mask wearing is also a major method for reducing spread. Masks have been shown to dramatically reduce the spread of large particles from the mouth. Basic cloth masks, therefore, are very good at protecting other people. They may also protect the wearer to a small degree, but they do not prevent breathing in viral particles. For that you need N95 masks (which block 95% of viral particles) or better.

Since the major route of transmission is through large droplets, the main mechanism to prevent spread is social distancing and wearing masks.

There is also the possibility of direct physical spread, mainly through direct contact. If an infected person touches their nose or mouth they can get viral particles on their hands, and if they then shake hands with someone else they can transfer the virus to their hands, and when that person scratches their nose they can catch the virus. The way to prevent this transmission is to not touch other people, and to frequently and effectively wash your hands and use hand sanitizer.

Indirect transmission through physical objects (called fomites) is also possible – through a doorknob, for example. The virus can survive on some surfaces for hours, and perhaps days, but this is considered a minor (but not insignificant) route of transmission. To avoid fomite spread, keep surfaces clean and wash public surfaces regularly with an antiseptic agent. Also, try to avoid touching such surfaces with your hands (elbows can be efficacious). In some settings wearing disposable gloves makes sense, but you still can’t touch your face with the gloves or that defies the purpose.

This brings us to aerosolized spread – the matter of the CDC controversy. Can viral particles survive in tiny droplets, the kind that can remain airborne for a long time? Larger droplets tend to fall quickly to the ground, and so mainly spread as projectiles from the mouth or nose. But aerosolized droplets can remain suspended in air, and travel much greater distances, through an indoor space. Cloth masks, as I said above, also don’t protect against breathing them in, only higher-end N95 masks or better do that effectively. Adding to the complexity is the fact that the distinction between “large” droplets and aerosols is somewhat arbitrary and settling time is actually a continuum based on droplet size.

One recent review found:

The case studies found worldwide indicate that the behavior of the SARS-CoV-2 virus has been unprecedentedly unique with more survival and viable rates in the air and believed to linger in the air for an extended period.

Another review was more stark:

The case studies that have come out in different countries have, with prima facie evidence, manifested that the airborne transmission plays a profound role in contracting susceptible hosts.

Yet another recent review concluded:

The authors believe that airborne transmission is possible and that HVAC systems when not adequately used may contribute to the transmission of the virus, as suggested by descriptions from Japan, Germany, and the Diamond Princess Cruise Ship. Previous SARS outbreaks reported at Amoy Gardens, Emergency Rooms and Hotels, also suggested an airborne transmission.

This seems to be the emerging consensus – there is reasonable evidence of airborne spread of COVID-19. All the reviewers also call for more research, but this is to dig deeper on the relative risk in different situations – in a home, building, hospital, car or airplane, and with different ventilation systems. But the fact that the risk of airborne spread is non-trivial now seems very likely.

What does this mean for our recommendations to minimize spread? Essentially it means no mass gatherings indoors. That social distancing alone is not adequate to prevent indoor spread. In high risk situations, like hospitals, N95 or better masks are required to protect workers. Good ventilation reduces, but does not eliminate, the risk.

It should also be pointed out that all of these protective measures need to be done concurrently. Having a good ventilation system, for example, does not mean you don’t need to wear masks or use hand sanitizer – these methods are all preventing different routes of spread. We need to simultaneously do everything.

But also, the realization that airborne spread is non-trivial should inform public policy in terms of school reopening and holding large indoor events. Further, people who are sick still need to isolate, which further means we need continued testing and contact tracing. COVID-19 is, overall, highly contagious, and the best method to prevent spread is simply isolation of anyone carrying the virus.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.