Some COVID-19 Patients Infect Many; Others Infect None

May 20, 2020 by Christen Aldrich

Filed under COVID-19 for Employers, COVID-19 for the Workforce

Last modified May 21, 2020

Infectious diseases tend spread in clusters, and with 5 million reported COVID-19 cases worldwide, big outbreaks were to be expected. An article published in Science Magazine describes how SARS-CoV-2, like two of its cousins, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), seems especially prone to attacking groups of tightly connected people while sparing others.

Most of the discussion around the spread of SARS-CoV-2 has focused on the average number of new infections caused by each patient. Without social distancing, this reproduction number (R) is about three. But in reality, some people infect many and others don’t spread the disease at all. Jamie Lloyd-Smith of the University of California, Los Angeles, who has studied the spread of many pathogens says: “The consistent pattern is that the most common number is zero. Most people do not transmit.” However, this is why in addition to R, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. The lower k, which transmits more, comes from a small number of people. Scientists have concluded that k for COVID-19 is somewhat higher than for SARS and MERS. 

An individual patients’ characteristics play a role in spreading the virus as well. Some people yield far more virus, and for a longer period of time, due to differences in their immune system or the distribution of virus receptors in their body. Some people also breathe out more particles than others when they talk. The volume at which they speak makes a difference, as does whether they are singing or simply speaking. It is suspected that singing may release more virus than speaking. People’s social behavior plays a role as well; having many social contacts or not washing your hands makes you more likely to pass on the virus.

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