‘Superspreading’ Drives Most COVID-19 Spread

• On March 10, a choir practice in Skagit County Washington was attended by 61 people, including one who had developed a “cold” three days prior to the practice. In the days following the 2.5 hour practice, 52 COVID-19 cases occurred (an infection rate of 87%). Three were hospitalized and two died from the disease.

• A couple of weeks before that choir practice, a Boston biotechnology company held a two-day leadership conference for about 175 of its world-wide executives. Apparently, someone was unknowingly infected and it spread to participants who took it back to their homes. At least 99 people from the meeting alone came down with COVID-19.

• About the same time, a funeral in Albany, GA was attended by >100 people. The virus was in attendance as well leading to a huge outbreak that spread into surrounding rural areas causing one of the nation’s largest local outbreaks of COVID-19.

• In Arkansas, an infected pastor passed the virus to more than 30 Sunday church attendees, three of whom died.

Epidemiology:

Somewhere between 10-20% of infected people are likely responsible for 80% of COVID-19 cases as recently described in Scientific American and reported in a study published in the Proceedings of the National Academy of Sciences. These “superspreaders” are typically asymptomatic and under the age of 60 according to investigators at Emory University’s Rollins School of Public Health.

Epidemiologists who study infectious diseases refer to the R₀, or “R naught,” value, which is a mathematical description of the reproduction number of an infectious disease. Seasonal flu has an R₀ of about 2, meaning that each infected person, on average, will spread the virus to two more people. The R₀ for Cov-2 is about 4, meaning it is twice as infectious as the flu. For comparison, the R₀ for measles, the most infectious virus we know, is 12-18. For the SARS pandemic in 2003, the early R₀ was about 2.75 but soon was brought down to <1 due to isolation strategies.

In addition to R₀, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. The lower k is, the more the transmission clusters, or comes from a small number of people. A 2005 Nature paper estimated that SARS had a k of 0.16. The estimated k for MERS, which emerged in 2012, is about 0.25. In the Spanish flu pandemic of 1918, in contrast, the value was about one, indicating that clusters played much less of a role in spreading the Spanish flu than it did in spreading the SARS and MERS coronaviruses.

Current estimates of k for CoV-2 vary. In January, researchers at the University of Bern concluded that k for COVID-19 is somewhat higher than for SARS and MERS. In a recent preprint, it was estimated that k for COVID-19 is as low as 0.1, which means that probably about 10% of cases lead to 80% of the spread.

Superspreading:

According to the CDC, super spreading is caused, in part, by individuals known as “superemitters,” who release more aerosol particles during speech than do their peers. Some people shed far more virus, and for a longer period of time, than others, perhaps because of differences in their immune system or the distribution of virus receptors and virus in their body.

Yet, other factors play a role in creating superspreader events. These are physical factors including crowd size, close contact, closed environment, and ventilation. Japanese researchers found that CoV-2 transmission was 18 times greater in a closed vs open-air space. Unsurprisingly, London researchers also found that the largest clusters of outbreaks were in indoor spaces like nursing homes, churches, schools, shopping areas, dormitories, prisons and ships.

It makes sense that an important factor in creating a superspreader event is the number of people involved. If a group of five friends meet, the chance of a superspreader event is much less than if 500 met. Plus, the chance of having an infectious person in the group is much reduced in the group of five vs 500. Hence, it makes sense to limit the size of gatherings in attempt to limit superspreader events.

Time also plays a key role in the virus spread. Various groups consider 10-15 minutes contact with an infected person to be the magic limit. This might help explain why grocery store cashiers, who only interact with customers for a couple of minutes, have not been hard hit with the virus, while meat-packing employees who work long shifts side-by-side, have been hard hit in some places.

Bottom line:

Reducing superspreading events by avoiding the “three C’s,” Closed spaces with poor ventilation, Crowded spaces, and Close contact settings, have a dramatic effect at reducing the R₀ of the virus, which, in turn, limits its morbidity and mortality. That is what the quarantine efforts have attempted to do. Certain countries around the world (S Korea and China for example) that have been very quick to identify and strictly control local outbreaks have been successful in reducing superspreading events. But, their strict measures are anathema to the psyche of the US. We prefer to deal more with the disease than the draconian restriction of our freedoms.

Other research reported in these pages has shown that liberal use of face masks can also reduce the R₀ value to <1, which is the bench mark for stopping an infectious disease. Could liberal use of face masks be the happy medium for the US? Could it be a way to avoid draconian social and life restrictions while limiting the spread of the virus until we have an effective vaccine?

I believe so.

Comments

[Ray Glew]

This was helpful. More specific about the details of how it spreads and it's variations in doing so.