Gone with the Wind: SARS-CoV-2 in the Air

This post was reviewed by Dr. Zachary Benet, one of our subject matter experts.

The paper we’ll be demystifying can be found here, if you’d like to follow along!

This research was conducted by scientists at Erasmus University Medical Center in The Netherlands.

TL;DR:

Researchers infected ferrets with SARS-CoV-2 and placed these ‘patient zero’ ferrets in a cage system so that they could spread the virus to other ferrets either through direct contact (ferrets in the same cage) or through the air (ferrets in a neighbouring cage). In their experiment, the direct contact ferrets got SARS-CoV-2 and so did the ferrets in neighbouring cages. This indicates that SARS-CoV-2 can be transmitted between ferrets, not only through direct contact but also through the air. The researchers highlight that their findings support the use of facemasks and social distancing.

Introduction:

COVID-19, the disease caused by the coronavirus SARS-CoV-2, has been tearing across the globe for the last several months. Researchers, medical professionals and other health experts all over the world have been racing against the virus with a single goal in mind- slow down the transmission. To prevent further spread we need to understand exactly how SARS-CoV-2 can jump from person to person. Current data suggests that the main way SARS-CoV-2 spreads is through respiratory droplets and physical contact.

There are two main ways that respiratory viruses can be transmitted through the air: respiratory droplets and aerosols. What’s the difference?

Respiratory droplets¹ are expelled out of the body when someone sneezes, coughs, or does activities like talking, singing and shouting. The researchers explain in the introduction of this paper that respiratory droplets are bigger than aerosols. Because of this they’re also heavier and don’t stay in the air very long before being pulled down by gravity. Because of this tendency to settle quickly, you have the highest chance of spreading viruses through respiratory droplets when you’re in close quarters with other people (usually within 1 meter).

Aerosols are smaller than respiratory droplets and they can stay in the air for a longer amount of time. Because aerosols can hang around in the air for a longer amount of time, they might be able to infect people further away. When a virus can be transmitted through aerosols, the WHO classifies it as ‘airborne’. Data has suggests that SARS-CoV-2 is spread mainly through physical contact and through respiratory droplets. That being said, that being said, there wasn’t that much concrete scientific evidence showing whether SARS-CoV-2 can be spread through the air- until now.

Other studies, like this one summarized by our author Manolya (Shi et al., 2020), have shown that ferrets can be infected by SARS-CoV-2. It’s also been shown that ferrets can spread SARS-CoV-2 to other ferrets through direct contact. The paper we are summarizing today was published early July in Nature Communications. This study provides scientific evidence of SARS-CoV-2 spreading from ferret to ferret both through direct contact and the air.

Experimental setup:

This study consisted of four independent experiments. What that means is that the experimental setup described below was done four separate times.

Here’s the setup for one experiment:

Ferrets were placed individually in cages. This means that each cage only had one ferret in it. These ferrets, called ‘donor ferrets,’ were then infected with a strain of SARS-CoV-2. Six hours after the exposure, a second ferret was added to each cage. This second ferret was called the ‘direct contact’ ferret because it has direct contact with the infected ferret.

The next day, another cage with another ferret was placed next to each of the existing cages. This loner ferret was called an ‘indirect contact’ ferret. The two cages were separated from each other by two steel grids that were ten centimeters apart. The only way for the virus to be transmitted from the cage containing the ‘donor’ ferret and the ‘direct contact’ ferret to the ‘indirect contact’ ferret would be through the air.

For clarity, here’s a helpful illustration of the cage setup. This is figure 1b from the paper.

The researchers collected throat, nasal and rectal swabs from each ferret. These swabs were tested for SARS-CoV-2 through RT-qPCR and virus titration.

Results:

First, the researchers checked if the ‘donor’ ferrets had been successfully infected with SARS-CoV-2. This was confirmed by the detection of long-term and strong viral shedding from these ferrets. Virus shedding is when infectious particles of a virus replicating inside the body is released into the environment.  This can be detected by throat, nasal and rectal swabs. Virus particles detected in the throat and nasal swabs could leave the body through actions like sneezing, coughing or breathing. Researchers found that the SARS-CoV-2 RNA levels were highest 3 days after the donor ferrets were exposed to the virus.

The donor ferrets infected their direct contact roommates in all four independent experiments. It took 1-3 days after exposure for virus RNA to be detected in the direct contact ferrets. Viral RNA continued to be detectable in the direct contact ferrets for up to 13-15 days after exposure.

Now for the kicker- in three out of the four independent experiments, the indirect contact ferrets were also infected. This indicates that SARS-CoV-2 was transmitted through the air to these ferrets since they were not in direct contact with any of their infectious friends. It took 3-7 days after exposure for SARS-CoV-2 RNA to be detected in the indirect contact ferrets, and the RNA continued to be detectable for 13-19 days post-exposure.

Next, the researchers looked at the virus shedding pattern in each category of ferrets.

The donor ferrets were initially exposed to a very high virus dose. In contrast, the researchers speculate that the indirect and direct contact ferrets would have received a lower infectious dose of the virus either through physical contact or through the air.  Even though the initial number of virus particles entering the ferrets were potentially very different, all three categories of ferret had very similar virus shedding patterns. This seems to imply that SARS-CoV-2 made more copies of itself very quickly and efficiently once entering the animals, no matter what the initial dose was.

Interestingly, in two of the four experiments they ran the indirect contact ferret seemed to have a much higher viral load at day three than the direct contact ferret. One other interesting result- the researchers did not isolate any infectious virus particles from the rectal swabs. This seems to suggest that viruses might be inactivated or destroyed when they go through the digestive system. That being said, the researchers caution that it’s too early to rule out transmission of SARS-CoV-2 via feces.

Discussion and Limitations:

This paper showed that SARS-CoV-2 can be transmitted between ferrets, not only through direct contact but also through the air. More importantly, the paper showed that air transmission of SARS-CoV-2 is efficient. The infected animals shed infectious virus particles for a long time!

A limitation of this experiment is that there’s no way to tell whether the air transmission of the virus was through respiratory droplets, aerosols or both. The two cages were placed only 10 cm apart, and both respiratory droplets and aerosols are capable of travelling that distance. On top of that, the direction of the airflow in their experiment is set up in a way that would blow respiratory droplets or aerosols from the ‘infectious’ cage into the other cage which amplifies the effect. Before the researchers can determine whether SARS-CoV-2 is spread by respiratory droplets or aerosols, more experiments are needed (such as the ones in this paper³ (Liu et al., 2020), summarized by Tristan here).

Another limitation of this experiment is the relatively small sample size (4 experiments). And of course, it’s always important to bear in mind that data seen in animal studies doesn’t always translate to humans.

This paper has given us the first experimental evidence that the SARS-CoV-2 virus can spread through the air. The researchers highlight that their findings support the use of facemasks and social distancing. Especially with businesses and workplaces opening up, it’s important to consider that SARS-CoV-2 can spread through the air, not just touch. Let’s stay safe-  stay six feet away, wear a mask, and socially distant as much you can- you owe to yourself and the people around you!

References:

1. Atkinson J, Chartier Y, Pessoa-Silva CL, et al., editors. Natural Ventilation for Infection Control in Health-Care Settings. Geneva: World Health Organization; 2009. Annex C, Respiratory droplets. Available from: https://www.ncbi.nlm.nih.gov/books/NBK143281/
2. Shi, J., Wen, Z., Zhong, G., Yang, H., Wang, C., Liu, R., He, X., Shuai, L., Sun, Z., Zhao, Y., Liang, L., Cui, P., Wang, J., Zhang, X., Guan, Y., Chen, H., & Bu, Z. (2020). Susceptibility of ferrets, cats, dogs, and different domestic animals to SARS-coronavirus-2. https://doi.org/10.1101/2020.03.30.015347
3. Liu, Y., Ning, Z., Chen, Y. et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 582, 557–560 (2020). https://doi.org/10.1038/s41586-020-2271-3