The Making of a Superspreader

Have you ever heard of Typhoid Mary? She was a private cook, a real person, elevated to mythical status for having infected more than 50 people with typhoid fever in the late 1800s and early 1900s. Three of those infected died, all without Mary herself ever having had symptoms of the disease. She is what epidemiologists call a superspreader.  

A superspreader is a person or event at the center of an outbreak. For most infectious diseases, 20 percent of the people are responsible for 80 percent of the spread. This is true for SARS, Ebola, and HIV. For COVID-19, the percentage is closer to 10 percent, which means superspreaders have an even greater influence in spreading COVID than they do for many other infectious diseases.

So what makes a superspreader a superspreader?

The first ingredient in the superspreader equation is the person’s individual biology, an immune system that can host a high viral load. In these individuals, the virus replicates and sheds virus more readily than in others for one of a couple reasons. If someone’s immune system is very tolerant, they may build up a lot of virus before developing symptoms. Emerging research appears to indicate that even those with average immunity are most likely to spread COVID in the 48 hours before symptoms arise. Alternatively, someone with a weak immune system may allow the virus to replicate and spread more readily. It’s not possible to know in advance if you or a family member is a potential superspreader.

A person can’t spread the virus to a large number of people unless they put themselves in contact with a group, so where they go is especially important.  Superspreaders go into a group of people. There is no minimum number of people for a superspreader event. It could be a 10-person dinner party or a 100-person wedding. Who the potential superspreader encounters is also important. If the people in the group have lower immunity or are at higher risk of becoming infected, the superspreader is more likely to infect more people.

Finally, what they do while in the group makes a significant impact. One of the first superspreader events of the COVID-19 pandemic happened at a biotechnology conference in Boston in February. The attendees had no idea COVID was a concern, so they shook hands and shared meals like it was 2019. Ninety-nine people became ill.

Mitigation efforts, geared to the specific disease, are helpful. For COVID-19, these mitigation steps include wearing a mask, staying 6 feet from others, washing your hands, and socializing primarily outdoors and for short spans of time. It should be noted, however, that some superspreaders are so infectious that they can spread the disease even while following mitigation steps.

Behaviors that increase the likelihood of spreading COVID in a group include singing, shouting, and breathing heavily, in addition to coughing and sneezing. In Washington state in March a superspreader at choir practice—standing closely with others singing for more than 2 hours—infected more than half of the 61 people in attendance.

Typhoid Mary was biologically unique. She went into people’s homes and cooked for them, without washing her hands. For typhoid, that was all she needed to do to meet all three criteria for superspreader status. As we approach the holidays, I encourage you not to become a superspreader yourself. We can’t know if you are biologically attuned. The only way to prevent being the center of an outbreak for sure is to avoid indoor gatherings, until we have a safe and proven vaccine that is freely available to all.

Donna Barron, RN, is the infection preventionist at Southwestern Vermont Medical Center in Bennington.