By JOHN RICHARD SCHROCK
In the pandemic Flu of 1918, everyone who came down with that especially virulent strain of influenza seemed to have an equal chance of spreading it to others. But for other contagious diseases, some who are infected appear to be major spreaders of the disease, while others do not pass it on despite being in contact with others. Although we are not five months into this pandemic, researchers are finding a few patients spread the disease more than others. These are “superspreaders.”
In the May 22 issue of the journal Science, the article “Case clustering emerges as key pandemic puzzle” adds a third measure of disease severity and transmission.
The first was the fatality rate, or the proportion of patients who contract the disease who then die. In the best of COVID-19 cases, that number is 6–10 times higher than for the seasonal flu, and in the more elderly population in Italy, it was much higher.
The rate of reproduction ( R ) measures how contagious a disease is; it is the average number of new infections that occur from contact with an infected patient. For COVID-19, that number averages about three. And any number above one indicates that the disease will spread more rapidly.
Now epidemiologists are finding that for some COVID-19 patients, there is no transmission to others. But a few patients infect a much larger number. Thus the R value average hides this difference between patients who are not shedding the virus, and the superspreaders.
And yes, epidemiologists have a number for superspreading: the dispersion factor (k). This ranges from zero to one, with one indicating an even dispersion where everyone infected has an equal chance of transmitting the disease. In the flu pandemic of 1918 that started in Kansas, any person who came down with the influenza virus was equally likely to infect others. But this pandemic is different.
In May 12, the CDC released their analysis of a superspreading event at a church in Mount Vernon in the state of Washington. Sixty-one people assembled for choir practice including one who thought he was suffering a cold. He did not yet know he had COVID-19. “In the following weeks, 53 choir members got sick, three were hospitalized, and two died....” A choir has been shown to be a very effective way to spread the virus in the many invisible droplets that are widely spread during singing.
The Science report continues with data collected by the London School of Hygiene & Tropical Medicine: nearly 800 cases from a Singapore dormitory, 56 cases from attending a Zumba class in South Korea, and 80 cases from a live music show in Osaka, Japan. All started with one or a few superspreaders.
Other serious coronavirus diseases also appear to be spread by select carriers. A 2005 paper in the journal Nature is cited where the highly fatal SARS coronavirus had a k of 0.16, indicating it was only transmitted by a few of those infected, but those few were highly contagious. While SARS was a coronavirus originating in bats that spread to humans via civet cats, MERS was the coronavirus that spread to humans via camels. MERS has an estimated k of 0.25, still transmitted in clusters but with a lower rate of superspreading.
The current estimate of k for the COVID-19 virus is 0.1 according to Adam Kucharski of the London School of Hygiene and Tropical Medicine. He simplifies this as “Probably about 10 percent of cases lead to 80 percent of the spread.”
More research is needed into why some patients lead to far more contagion than others. Experts who have worked with the different Ebola and HIV point to the main reason for these transmission clusters is that SARS-CoV-2 (the virus that causes COVID-19) is spread by aerosol transmission. There are differences in each patient’s immune system. --And how many viruses are shed. –And for how long.
Chinese researchers traced 318 clusters and found only one was transmitted outdoors. A Japanese study likewise found “...that the risk of infection indoors is almost 19 times higher than outdoors.”
With this type of medical detective work and research, an intelligent population can begin to resume life in a rationale and compassionate manner.
. . .
John Richard Schrock has trained biology teachers for more than 30 years in Kansas. He also has lectured at 27 universities in 20 trips to China. He holds the distinction of “Faculty Emeritus” at Emporia State University.