We are getting to one full year of the COVID-19 pandemic, which does feel like a long time, but it’s important to remember a couple of things. First, this is a relatively new virus and we still have a great deal to learn about it. Also, we are still in the middle of this pandemic. In fact, we are at the beginning of a third peak which looks to be bigger than the first two. It increasingly looks like this pandemic has a long way to go, and we will likely need an effective vaccine to get back to some semblance of pre-pandemic normal.
One of the aspects of COVID-19, and the SARS-CoV-2 virus that causes it, that we are still studying is the epidemiology – how does the virus spread? The more we know about how this virus spreads, the better we will be able to take steps to minimize it. For example, we now know that the primary way the virus is spread is through droplets which are spread directly from one person to another. This is why mask-wearing and social distancing are effective, and remain the primary methods of reducing spread. But we also know that the virus can spread through aerosolized smaller droplets, which means they can be carried long distances and spread from one person to the next just by being in the same enclosed space. This means we need to avoid large indoor crowds, and that indoor spaces should be well ventilated. Physical spread through direct contact or indirect contact via fomites (like door handles) plays a smaller role, but is still important, necessitating hand washing and wiping down surfaces.
One question researchers have been tracking is the role of so-called superspreader events, by definition when one person spreads the virus to more than six other individuals. David Gorski wrote about this question previously, but we have already learned more since then. Even three months ago we knew that the virus does not spread uniformly. The basic reproduction number for COVID-19 – on average how many people will each infected person spread the virus to – is three. That’s actually a high number and makes this a very infectious illness. But this obviously does not mean that everyone who catches COVID-19 will spread it to three other people; this is just an average. Some people may spread the virus to no one while others spread it to many people.
A new study adds some further insight into this question. Researchers at MIT modeled the spread of SARS-CoV and SARS-CoV-2 based on published reports of superspreading events. These are considered extreme events, because they deviate significantly from the average. But the question is: how common are such events and how much of a role do they play in the overall spread of the virus? In mathematical terms, how “fat” is the statistical tail? This refers to a graph of the numbers of people that each infected person spreads the virus to. There will be a peak around 3 with a tail to the left (down to zero) and the right. The fatter the right tail, the more common are extreme spreader events where one person spreads the virus to many people.
Their data confirms what was previously suspected – the graph of COVID-19 spreaders has a fat tail, with extreme superspreader events being fairly common. As David pointed out in his previous article, it was estimated that between 10-20% of those infected are responsible for 80% of new cases. The new models confirm that basic picture, with most cases coming from relatively few superspreader events (if you count all the subsequent people infected by those who were infected at a superspreader event).
This information also tells us how best to limit the spread of this virus – avoid superspreader events. This means avoiding large gatherings of people, especially indoors. The researchers suggest that gatherings in hot zones should be limited to 10 people or fewer. Smaller gatherings also make it easy to social distance. When possible such events should be outdoors.
But it is important to recognize that being outdoors is not a panacea. Often I hear people say that they can relax their mask-wearing and social distancing because they are outdoors. The White House superspreader event was outdoors, just giving one example of why this is not true. Being outdoors limits aerosolized spread, but does not limit large droplet spread, which is the main vector of spread anyway. If you are talking to someone a few feet away without a mask, you are showering them in droplets from your mouth, and if those droplets are teeming with SARS-CoV-2 they will likely catch it, whether or not you are outdoors.
Even before, but certainly with this new study, it is fairly clear that one of the most effective methods for minimizing the spread of the COVID-19 pandemic is to limit large gatherings. This one step can prevent superspreader events, which can reduce the spread of the virus by as much as 80%. This is inconvenient, but a lot less inconvenient than getting COVID-19. We may just have to accept this reality until we can widely distribute an effective vaccine.
