Computational projection of virion transmission rates to the lower airway from the initial SARS-CoV-2 infection at the Nasopharynx

While the nasopharynx stands out as the dominant initial infection site for SARS-CoV-2, the physiological mechanism launching the lower airway infection is still not well-understood. Based on the speed of infection progress, it is thought that the nasopharynx acts as the seeding zone for subsequent contamination of the lower airway via aspiration of virus-laden boluses of nasopharyngeal fluids. We examine the plausibility of this transport process through computational fluid mechanics models of steady and forced breathing in five tomographic airway reconstructions, thereby quantifying the nasopharyngeal liquid volume transmitted to the lower airspace in each aspiration. Our model predicts 2-4 aspirations during an 8-hour sleep cycle, consistent with prior experimental data. Extending the numerical trends on aspiration volume to earlier records on aspiration frequency indicates a total aspirated nasopharyngeal liquid volume of 0.3±0.76 ml/day. Using sputum viral loads for hospitalized COVID-19 patients, we then estimate the number of virions transmitted daily to the lungs via nasopharyngeal liquid boluses. For peak sputum viral load, the number is 7.1 × 108±1.8 × 109 virions/day, well in excess of the estimated minimum infectious dose for SARS-CoV-2. These findings provide a mechanism for the progression of SARS-CoV-2 infection of the nasopharynx to the COVID-19 disease within a patient, and point to dysphagia as one of the potential underlying risk factors for adverse outcomes.