Finding the infectious dose for COVID-19 by applying an airborne-transmission model to superspreader events
Mara Prentiss1, Arthur Chu2, Karl K Berggren3
1Department of Physics, Harvard University, Cambridge, MA, United States of America.
Abstract:
We probed the transmission of COVID-19 by applying an airborne transmission model to five well-documented case studies-a Washington state church choir, a Korean call center, a Korean exercise class, and two different Chinese bus trips. For all events the likely index patients were pre-symptomatic or mildly symptomatic, which is when infective patients are most likely to interact with large groups of people. Applying the model to those events yields results that suggest the following: (1) transmission was airborne; (2) superspreading events do not require an index patient with an unusually high viral load; (3) the viral loads for all of the index patients were of the same order of magnitude and consistent with experimentally measured values for patients at the onset of symptoms, even though viral loads across the population vary by a factor of >108. In particular we used a Wells-Riley exposure model to calculate q, the total average number of infectious quanta inhaled by a person at the event. Given the q value for each event, the simple airborne transmission model was used to determined Sq, the rate at which the index patient exhaled infectious quanta and N0, the characteristic number of COVID-19 virions needed to induce infection. Despite the uncertainties in the values of some parameters of the superspreading events, all five events yielded (N0∼300-2,000 virions), which is similar to published values for influenza. Finally, this work describes the conditions under which similar methods can provide actionable information on the transmission of other viruses.
Insights
Airborne transmission models confirm COVID-19 spread through aerosols, even from pre-symptomatic individuals. Superspreading events don't require high viral loads, with consistent infectious doses found across cases.
Area of Science:
- Epidemiology
- Infectious Disease Transmission
- Public Health
Background:
- The transmission dynamics of COVID-19, particularly concerning airborne spread and superspreading events, remain a critical area of research.
- Understanding the role of pre-symptomatic and mildly symptomatic individuals in viral transmission is crucial for effective public health interventions.
Purpose of the Study:
- To investigate the transmission of COVID-19 using an airborne transmission model.
- To analyze five documented case studies, including a church choir, call center, exercise class, and bus trips, to understand transmission patterns.
- To determine the infectious dose of SARS-CoV-2 and evaluate the characteristics of superspreading events.
Main Methods:
- Application of an airborne transmission model, specifically the Wells-Riley exposure model, to analyze five real-world COVID-19 case studies.
- Calculation of the total average number of infectious quanta inhaled (q) by individuals at each event.
- Determination of the rate of infectious quanta exhalation (Sq) by the index patient and the characteristic number of virions needed to induce infection (N0).
Main Results:
- Airborne transmission was identified as the primary mode of spread in all analyzed events.
- Superspreading events were found not to necessitate an index patient with an unusually high viral load.
- The estimated infectious dose (N0) for COVID-19 was consistent across all five events (approximately 300-2,000 virions), similar to influenza.
Conclusions:
- COVID-19 transmission is predominantly airborne, facilitated by pre-symptomatic or mildly symptomatic individuals.
- The findings suggest that superspreading events are influenced by factors beyond just high viral load, with a consistent infectious dose.
- The applied modeling methods can offer valuable insights into the transmission of other viral respiratory illnesses.
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