The good, the bad, and the ugly of COVID-19 superspreaders.
During a disease outbreak, you might well think that every infected individual has an equal chance of spreading the disease to others. However, for most contagious diseases and for COVID, this is not the case. Various epidemiological studies find that about 20% of individuals within any given population are thought to contribute at least 80% to the transmission of a potential pathogen.
Who or what are superspreaders?
“Superspreader” is a term that refers to individuals who disproportionately infect a high number of individuals and likely drive the speed and severity of an outbreak. Some individuals are just more infectious than others, and some are just more vulnerable, leading to heterogeneous transmission. We also know almost nothing about how to measure and predict these individual factors.
Recently, the term “superspreader” has come to be applied more broadly and now encompasses events, such as the Rose Garden Ceremony, as well as to settings, such as the Diamond Princess cruise ship. We have learned much more about what creates superspreader events and settings than we know about superspreader individuals themselves, but we still know little about how these factors interact to create the worst outbreaks.
For COVID-19, 10% to 20% of people are estimated to be responsible for 60% to 80% of total infections. This estimate dramatically points to how COVID-19 is highly dependent on specific individuals and how they behave – whether they take precautions, the events they attend and the settings in which they contact others. Therefore, we cannot solely rely on a single Ro (“R-naught” or reproductive number) to estimate how contagious this virus is for any particular individual or group behaving in specific ways.
Do super-spreaders affect Ro?
In short, yes. The Ro value estimates how contagious an infectious disease is. Essentially, any Ro greater than one indicates the epidemic is continuing to grow. On the other hand, an Ro below one indicates the virus is being curtailed. For example, an Ro value of two would indicate that each individual will infect two others on average. Superspreaders (or lack thereof) can drastically affect the Ro. Although the Ro value can be skewed by superspreaders, it is still a good starting point to assess the overall trajectory of an outbreak to help plan potential interventions that should be taken.
Is COVID-19 the first disease to spread through superspreaders?
NO! In fact, many of us will remember learning about Mary Mallon, an Irish immigrant in the U.S. in 1884. She was regarded as the first “healthy carrier” (aka asymptomatic carrier) of Salmonella Typhi, which ultimately got her the nickname of “Typhoid Mary.” During her time in the U.S., she was responsible for the infection of over 100 people and five deaths. Additionally, many other diseases that we know of are thought to spread via superspreaders, which were considered to be drivers of the MERS and SARS (2002–2003) outbreaks.
The good news about super-spreaders?
Very few of us will be a superspreader and, for those unfortunate to become one, it will not be for long. COVID’s infectious period lasts days; it’s not like Typhus, which can last for months untreated. Even “COVID long-haulers” with lingering symptoms have proven to not have the capacity to infect others beyond the first seven to 10 days. Only a limited set of individuals, events and settings have the potential to become superspreaders, so when we figure out what they are, we can dramatically reduce the need for the enormous inconvenience of the sort of blanket mandates (resented or rejected by so many) that policymakers impose, e.g. Britain’s second month-long lockdown currently underway.
The bad news is that we know very little about what causes some individuals to become superspreaders, and most are only discovered in hindsight. Being able to identify superspreaders before they wreak havoc would prove highly beneficial. These individuals, together with the most vulnerable, should receive a future vaccine first as they could potentially do the most harm to others.
In the meantime, we do know that close contact with the viral aerosols in the exhaled breath of infected individuals is to be avoided at all costs. That means avoiding enclosed and ill-ventilated spaces where shouting, talking or singing occurs in close quarters.