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COVID-19: Transmission


The estimated reproduction number (R0) for SARS-CoV-2 ranges from 2.2 to 5.5, meaning one person can potentially transmit the disease to 5–6 people, making it highly transmissible. 

Three main routes of transmission have been proposed:

  1. The 'fomite' path, through touching a surface that contains the SARS-CoV-2 virus. That can transfer the virus onto your hand, and then you can infect yourself by touching your mouth, nostrils, or eyes.
  2. The ‘large droplet’ or ‘ballistic droplet’ path. Droplets are particles of saliva or respiratory fluid (larger than about 100 μm, with 1 μm = a millionth of a meter) that are expelled from infected individuals when coughing, sneezing, and to a lesser extent, talking. They infect by impacting on the mouth, nostrils, or eyes. If they don’t hit someone, they fall to the ground in 1-2 m (3-6 ft).
  3. The ‘aerosol’ path. Aerosols are smaller (less about 100 μm) particles of saliva or respiratory fluid. They can linger more in the air, from tens of seconds to hours, and can travel longer distances. They infect by being inhaled through the nose or mouth, or (less likely) by deposition on the eyes. 

The relative importance of these modes of transmission is uncertain and open to debate

It is likely that some individuals are more contagious than others. This may be due to a higher viral load at the onset of symptoms, to higher emissions of respiratory particles, or (likely) to both. Viral RNA shedding is higher at the time of symptom onset and declines after days or weeks. The RNA of the virus has been identified in respiratory tract specimens 1-2 days before the onset of symptoms and it can persist for up to eight days in mild cases and for longer periods in more severe cases, peaking in the second week after infection.  

Prolonged viral RNA shedding has been reported from nasopharyngeal swabs (up to 67 days among adult patients) and in faeces (more than one month after infection in paediatric patients). Infectious virus has been detected up to day eight post disease onset using cell culture based systems.

Late viral RNA clearance (≥15 days after illness onset), is associated with male sex, old age, hypertension, delayed admission to hospital, severe illness at admission, invasive mechanical ventilation, and corticosteroid treatment.

 

Seasonality

Human coronavirus infection rates show peaks in the winter months, similar to influenza and human respiratory syncytial viruses (RSV) and are therefore, according to their seasonality, classified as winter viruses. Low temperature and dry air impair and disrupt the integrity of the epithelial layer of the lungs, which might explain the winter seasonality of respiratory viruses. 

Other factors that might contribute to transmission are increased indoor activities during the winter months, which increases susceptible host proximity.  Such behavioural factors have been implicated in other winter viruses such as influenza. Whether SARS-CoV-2 will show similar seasonality remains to be seen.


References