Transmission of SARS-CoV-2

Somedays it feels like an eternity, but is has been just nine months since the first case of SARS-CoV-2 was detected in the U.S. on January 19, 2020.[1]  Despite this rather short timeframe, there has been an avalanche of medical literature dealing with COVID-19; my PubMed query on “COVID-19” or “SARS-CoV-2” today returned an impressive 63,481 articles.  The challenge for clinicians, public health practitioners, and policymakers is simply keeping up.  Well-constructed reviews of recent literature can help.

The recent offering in Annals of Internal Medicine by Meyerowitz and colleagues provides a needed overview of the current thinking on SARS-CoV-2 transmission.[2]  Five central topics are approached and summarized below:

• Environmental viability of the virus
  • viable virus has been isolated for up to 3 hours in aerosols
  • viable virus has been isolated for up to 72 hours on surfaces
    • longest persistence is on plastics and stainless steel
  • virus is stable at lower temperatures
  • it is rapidly inactivated at temperatures ≥ 70^oC (158^oF)                
• Viral and host factors affecting transmission
  • susceptibility appears to be less in children < 10 years
  • increased transmission occurs with the D614G mutation
• Modes of transmission
  • respiratory transmission through droplets is the most common mode of transmission
  • proximity and duration matter because larger droplets (≥ 5µm) fall within about 6 feet
  • air circulation and ventilation is important
  • sexual, fecal-oral, and blood-borne transmission has not been demonstrated
  • whereas there is no evidence for direct contact and fomite transmission, consistent hand hygiene and use of chlorine or ethanol disinfectants for surfaces have been associated with lower transmission
  • SARS-CoV-2 replicates in cats and ferrets;
    • there is no evidence for transmission from pets to people
  • vertical transmission rarely occurs
• Period of infectiousness
  • individuals typically have a 5.2 day incubation period
  • individuals with mild-moderate symptoms rarely shed viable virus after 10 days
  • shedding is increased, however, in those with severe symptoms and/or hospitalization
• Population transmission dynamics
  • R_o (number of secondary cases per case) is estimated at 2 to 3
  • spread within a population is heterogeneous: a few people (superspreaders) transmit to many other; most people spread little
  • transmission within households is relatively high

My take home from this review: distancing, masking, hand hygiene, surface decontamination, ventilation, isolation of those testing positive for 10 days after symptom onset, and quarantining if exposed continue to be good practices to prevent the spread of SARS-CoV-2.