When Singing Kills

In this post, I'll look at what we can learn from a COVID-19 "super spreader" event linked to choir practice in Washington State at the beginning of March. Here is a picture from that summarized what happened:

Of 61 people who attended the 2.5 hour choir practice, 52 got infected with COVID-19. In all likelihood, one person who developed cold-like symptoms three days before the choir practice infected 51 others within two and a half hours. Two of these people died of COVID-19.

A "super sneezer" event?

It is "general knowledge" that COVID-19 primarily spreads through large droplets  - when someone sneezes or coughs directly onto you, or when droplets from his sneezing or coughing land on surfaces that you later touch. I am calling this "general knowledge" because many "authorities" have states this; that includes the CDC and WHO. The "6 feet distance" and the "wash your hands often" rules are based largely on this assumption: large droplets quickly fall to the ground, and don't travel further than a few feet.

However, many scientists have pointed out that COVID-19 is very likely to also spread through aerosols: smaller droplets that can remain airborne for much longer, and travel larger distances. In the "Washington Choir Superspreader Event", it is very likely that a large number of the COVID-19 infections happened through aerosols, and over distances larger than 6 feet. This conclusion is supported by everything we know about aerosol droplet generation and the stability of the new corona virus in such droplets.

But for the sake of argument, let us look if "large droplet" transmission can explain how one person infected 51 others in less than 3 hours. The article from the local health department explains that choir was seated in 6 rows of 20 chairs each, spaced 6-10 inches apart, with a center aisle. Let's assume our "infector", patient 0, was seated somewhere in the middle of a row towards the back. A chair is about 2 feet wide, so there are 2 people sitting next to him on each side - 3 if we are generous. The person sitting in from of him is about 3 feet way, and the two people next to him are also within a 6 ft radius. The person two rows ahead of him is roughly 6 feet away, but we'll also add him. That makes a total of 3+3+3+1 = 10 people within a six-foot radius to the front and sides. Any droplets from coughing and sneezing generally travel forward, but we even if we add three people who sit behind patient 0, we are still at only 13 people within a 6-foot radius. At some point, the choir actually broke into 2 separate sub-groups. If we assume that patient 0 was surrounded by a different subset of people there, that makes 26 people who were within his 6-foot radius.

So to cause all observed 51 infections, patient 0 would have had to "land" droplets on every single one of the people within his 6-foot radius, even the people to his side and behind him. But then, he would also have had to infect a similar number of people during much shorter encounters during the breaks, or when returning the chair. Note that the article stated that "No one reported physical contact between attendees", and that many of the attendees did not eat the snacks offered during the break.

In theory, it is possible that patient 0 did infect 51 others through large droplets - but it seems extremely unlikely. Even if he coughed and sneezed a lot during choir practice, it is hard to see how he would have infected more than the two people sitting next to him; perhaps a couple of people sitting in front of him; and perhaps a few more people during the break.

The simpler explanation: aerosol droplets

 It seems much more plausible that this event is an example of a "superspreader event" where COVID-19 spread primarily through aerosols. As I explained in my last post, scientists have shown that one minute of regular speech can release aerosol droplets with 1,000 - 5,000 virus particles. Loud speech has been shown to generate 5 to 10-fold more droplets. I am not aware of any study that specifically looked at singing, but every choir I have ever seen certainly would qualify for the "loud" category.

During two hours of singing, an average infected person would have released somewhere between 100,000 and half a million virus particles in aerosol droplets that can remain airborne for at least 8-14 minutes, and longer with any air movement. The article mentions "superemitters" who release an order of magnitude more droplets, which would increase the number of virus particles in the air to more than a million. However, the article completely ignores an important study that shows a huge variation in viral load between individuals: some individuals have more than 100 times the average number of virus particles in saliva or throat swabs. That would increase the number of released virus particles to 10 million to 50 million, or more.

The article does not mention the size of the choir practice room, but we can make a guess: if the room was about 70 ft (20 m) wide and long, and 8 ft (2.5 m) high, then the total air volume was 1,000 cubic meters. If we assume even distribution of the virus particles, that gives about 10,000 to 50,000 virus particle per cubic meter. The average person breathes about 0.6 cubic meters of air per hour, so roughly a cubic meter in 2 hours. According to the calculation above, he may have breathed in 10,000 to 50,000 virus particles - almost certainly way more than enough to get infected.

Overall, it seems much more likely that the primary source of transmission in this instance was by aerosol particles. Sure, there is "no proof", but neither is there any proof of large droplet or "fomite" transmission here. But the numbers give a very clear indication what is more likely: aerosol transmission.

The take home lesson: avoid large, loud groups

The more people there are, the more likely it is that someone is infected with COVID-19, and that there is a "superemitter". The more and the louder people talk, sing, or shout, the more virus they set free to search new hosts. If this happens indoors, the aerosol droplets can hang around for a long time; even after settling down on the ground or surfaces, they act like fine dust, and just walking by can put them back in the air. Outdoors, chances are better that aerosol droplets will be moved away quickly - but if the person behind you in the sports stadium is yelling for his team just as you turn around, you may breath in plenty of virus to get infected. The same can happen if you stand downwind of someone on the beach.. or just sunbath. Remember that aerosols travel farther than 6 feet!

Keeping distance still helps, especially outdoors or in well-ventilated areas, since aerosol droplets will be most concentrated closer to the speaker. Facemasks also help, even if they capture only half or three quarters of aerosol droplets; they also keep your "aerosol cloud" closer to you by slowing it down. But you should remember that facemasks generally let a lot of small droplets through - so keep your distance, too!

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Note added 6/17/2020: Scientists have been able to obtain the seating chart for the Washington Choir event, and concluded that aerosol transmission is the only possible explanation for the vast majority of infections. The study also outlines the possible effects of increased ventilation, and challenges in avoiding COVID-19 infections in restaurants and indoor events.