Covid-19 sars-coV-2 Lisa Gilbert, md, faafp, ctropMed 3/17/20 covid-19 sars-coV-2 2019-nCov hcoV-19

SARS-Cov-2 Transmission

• Novel: No herd immunity, No antibodies cross-reacting
• Incubation 2-14 days (outlier 27 days)
• Symptom onset median: Day 5-6 from exposure
• Doubling time: 6-7 days
• High viral shedding occurs early in disease course, even those with mild symptoms
• Prolonged shedding noted (unlikely reinfection)
• ? Up to 23% of transmissions due to pre-symptomatic cases in Shenzen
• True asymptotic cases apparently only 1% per WHO?
• Viral load apparently still high
• Apparently infectious?

https://cmmid.github.io/topics/covid19/control-measures/pre-symptomatic-transmission.html

https://www.who.int/docs/default-source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf

SARS-Cov-2 Transmission

• Respiratory droplets (large - 3 ft, medium - 6 ft)
• Hand-to-mucus-membrane contact – sticks to skin easily!
• T-zone: eyes, nose, mouth vulnerable
• Viable for 3 days on solids (plastics, porcelain, steel); ~24 hours cardboard, dependent also on temperature/humidity; 3 hours if aerosolized
• Airborne – likely not airborne with cough? But certainly possible with intubation, non-invasive positive pressure ventilation, high flow O2, nebulizer, suctioning
• ?Fecal/oral? – viral shedding present in stool and diarrhea is common

https://www.medrxiv.org/content/10.1101/2020.03.09.20033217v1.full.pdf

https://www.ajicjournal.org/article/S0196-6553(16)30531-4/pdf

Seasonality and Mutation Rate

• We don’t know.
• Some CoV are seasonal in northern hemisphere; MERS is not.
• High disease burden and outbreaks obscure seasonality.
• Typical viral mutation rate; see NextStrain for real time gene sequencing: https://nextstrain.org/
• Major strain development: L type (more virulent) and S type (wild type/apparent first strain). Lethality difference unclear.
• Normally viruses become more benign over time. However, delayed symptom onset, delayed time to death, poor access to diagnosis with isolation, contact tracing and quarantine makes deadly strains persist.