Last week, the Centers for Disease Control and Prevention posted—then removed—updated guidelines about airborne transmission of SARS-CoV-2, the coronavirus that causes COVID-19. These guidelines indicated that there was good evidence that the virus can travel further than six feet through the air (the current presumed safe physical distance), especially in indoor settings without good ventilation.
According to the CDC, the updated guidelines were a draft that was accidentally posted too early. But the move left the public and politicians alike wondering if this was another case of political interference with a vital public health institution.
So what do we actually know about airborne transmission?
When we breathe, talk, sing, or cough, we produce respiratory particles of a variety of sizes. If a particle is five micrometers or larger, it’s considered a droplet. If it’s smaller than that, it’s an aerosol. The larger, heavier droplets that we spew will fall to the ground first—usually within about six feet or less. Smaller particles can travel farther or hang in the air like smoke.
“When the CDC and the WHO say airborne, they almost always mean aerosolized transmission,” says Samuel Scarpino, an assistant professor at Northeastern and head of the Emergent Epidemics Lab. “However, the colloquial use of airborne would probably also include small or large droplet transmission.”
We know this coronavirus can be spread through contact with larger droplets. The question is, can these smaller particles, known as aerosols, also transmit the virus?
“We have an increasingly large body of anecdotal evidence that is all pointing in the same direction, which is that the virus can be transmitted via aerosol,” Scarpino says.
In July, 239 scientists from 32 countries wrote an open letter to the World Health Organization, urging officials to recognize that the coronavirus is airborne. The WHO has since updated its guidelines to include airborne transmission, although it still places the majority of the emphasis on droplets.
Superspreader events, such as the one that occurred at a choir rehearsal in Skagit, Washington in March, are part of the evidence indicating the virus can spread through aerosols. Despite some basic safety measures, such as avoiding physical contact and keeping some distance, a single symptomatic person likely spread the virus to 52 people during the two and a half hour rehearsal.
There is also evidence from laboratory studies showing that the virus can be carried in the aerosolized particles, and that airborne transmission can spread the virus between both ferrets and hamsters.
But we don’t have the smoking gun of a study demonstrating a person being infected by aerosol transmission in a laboratory-controlled setting. And we aren’t going to get it.
“Unless you do human trials, which are completely unethical in this case, it’s very hard to convincingly demonstrate that aerosol transmission is happening, as opposed to the many other possible routes that someone could have gotten infected,” Scarpino says. “We’re in a situation where the laboratory evidence is suggestive, but not as strong as it is for other diseases that we know are dominated by aerosol transmission.”
Does this change what I should be doing to keep myself and those around me safe?
You should still be wearing a mask.
“We know that mask wearing is highly effective,” Scarpino says. “If it was just aerosol transmission, cloth masks would be pretty nearly worthless and that’s not the case. So certainly, small and large droplets still matter.”
Just because aerosol transmission is possible, doesn’t mean it happens all the time. It could be rare, Scarpino says. This virus tends to spread through superspreader events, meaning that a large percentage of infections come from a small number of people. A study of the virus’ spread in Hong Kong estimated that 80 percent of infections were being caused by just 20 percent of people with the virus.
“You could imagine a situation where a small number of the largest superspreading events are due to aerosol transmission,” Scarpino says. “So even if it’s uncommon, when it happens, and you have one of those individuals in an enclosed space, you end up with a huge number of secondary infections.”
So it would be sensible to modify recommendations to limit the size of indoor gatherings, especially in places with poor ventilation. But definitely don’t stop washing your hands, keeping your distance, and wearing the all-important mask.
Why doesn’t the CDC just say all that?
“By their very nature, these representative bodies like the CDC need to be a little bit slower around digesting the scientific evidence and making sure there’s consensus in the scientific community before they really put out a big announcement,” Scarpino says.
Especially at a time when everyone is paying attention, the CDC and similar organizations need to get the messaging right.
But there are political ramifications associated with acknowledging aerosol transmission, Scarpino says, especially as we approach the election. The large political rallies that Donald Trump has favored in the past, especially those being held indoors, for long periods of time, would be off the table.
“Since we know that the Trump administration has manipulated the CDC’s messaging in the past, I think we have every reason to be very cautious around what motivated this change that we saw,” Scarpino says.